 0. An electronic musical instrument comprising: a delay time extracting means for storing a plurality of first delay times comprising a period that changes depending on a sounding volume and is a period after execution of a sound-producing instruction until generation of a string-striking sound and a plurality of second delay times comprising a period that changes depending on the sounding volume and is a period after execution of the sound-producing instruction until generation of a shelf board impulsive sound, and extracting a first delay time and a second delay time corresponding to a sounding volume specified by a sound-producing instruction operation from among the stored plurality of first delay times and second delay times; and a musical tone generating means for generating the string-striking sound having a pitch specified by the sound-producing instruction operation when the first delay time extracted by the delay time extracting means has elapsed after execution of the sound-producing instruction, generating the shelf board impulsive sound when the second delay time extracted by the delay time extracting means has elapsed, and generating a musical tone having a combination of the generated string-striking sound and the shelf board impulsive sound. 0. An electronic musical instrument comprising: a delay characteristics storing means for storing a plurality of first delay times comprising a period that changes depending on a key-pressing strength and is a period after execution of a key press until generation of a string-striking sound and a plurality of second delay times comprising a period that changes depending on the key-pressing strength and is a period after pressing a key until generation of a shelf board impulsive sound; an extracting means for extracting a first delay time and a second delay time corresponding to a key-pressing strength generated by a key-pressing operation from the delay characteristics storing means; a string-striking sound generating means for generating the string-striking sound having a pitch depending on the key-pressing operation when the first delay time extracted by the extracting means has elapsed after pressing the key; a shelf board impulsive sound generating means for generating the shelf board impulsive sound when the second delay time extracted by the extracting means has elapsed after pressing the key; and a musical tone generating means for generating a musical tone having added thereto the string-striking sound generated by the string-striking sound generating means and the shelf board impulsive sound generated by the shelf board impulsive sound generating means. 0. An electronic musical instrument comprising: a delay characteristics storing means for storing a plurality of types of delay characteristics having a plurality of first delay times comprising a period that changes depending on a key-pressing strength and is a period after execution of a key press until generation of a string-striking sound and a plurality of second delay times comprising a period that changes depending on the key-pressing strength and is a period after pressing the key until generation of a shelf board impulsive sound; a selecting means for selecting any of the plurality of types of delay characteristics stored in the delay characteristics storing means; an extracting means for extracting a first delay time and a second delay time corresponding to a key-pressing strength generated by a key-pressing operation from the delay characteristics storing means within the delay characteristics selected from the delay characteristics storing means by the selecting means; a string-striking sound generating means for generating the string-striking sound having a pitch depending on the key-pressing operation when the first delay time extracted by the extracting means has elapsed after pressing the key; a shelf board impulsive sound generating means for generating the shelf board impulsive sound when the second delay time extracted by the extracting means has elapsed after pressing the key; and a musical tone generating means for generating a musical tone having added thereto the string-striking sound generated by the string-striking sound generating means and the shelf board impulsive sound generated by the shelf board impulsive sound generating means. 0. A computer program product for an electronic musical instrument stored on a computer-readable medium and executed by a computer, comprising the steps of: a delay time extracting process for storing a plurality of first delay times comprising a period that changes depending on a sounding volume and is a period after execution of a sound-producing instruction until generation of a string-striking sound and a plurality of second delay times comprising a period that changes depending on the sounding volume and is a period after execution of the sound-producing instruction until generation of a shelf board impulsive sound, and extracting a first delay time and a second delay time corresponding to a sounding volume specified by a sound-producing instruction operation from among the stored plurality of first delay times and second delay times; and a musical tone generating process for generating the string-striking sound having a pitch specified by the sound-producing instruction operation when the first delay time extracted by the delay time extracting means has elapsed after execution of the sound-producing instruction, generating the shelf board impulsive sound when the second delay time extracted by the delay time extracting process has elapsed, and generating a musical tone having a combination of the generated string-striking sound and the shelf board impulsive sound. 0. A computer program product for an electronic musical instrument stored on a computer-readable medium and executed by a computer, comprising the steps of: an extracting process for storing a plurality of first delay times comprising a period that changes depending on a key-pressing strength and is a period after is a period after execution of a key press until generation of a string-striking sound and a plurality of second delay times comprising a period that changes depending on the key-pressing strength and is a period after pressing a key until generation of a shelf board impulsive sound, and extracting a first delay time and a second delay time corresponding to a key-pressing strength generated by a key-pressing operation from among the stored plurality of first delay times and second delay times; a string-striking sound generating process for generating the string-striking sound having a pitch depending on the key-pressing operation when the first delay time extracted by the extracting process has elapsed after pressing the key; a shelf board impulsive sound generating process for generating the shelf board impulsive sound when the second delay time extracted by the extracting process has elapsed after pressing the key; and a musical tone generating process for generating a musical tone having added thereto the string-striking sound generated by the string-striking sound generating process and the shelf board impulsive sound generated by the shelf board impulsive sound generating process. 0. A computer program product for an electronic musical instrument stored on a computer-readable medium and executed by a computer, comprising the steps of: a selecting process for storing a plurality of types of delay characteristics having a plurality of first delay times comprising a period that changes depending on a key-pressing strength and is a period after execution of a key press until generation of a string-striking sound and a plurality of second delay times comprising a period that changes depending on the key-pressing strength and is a period after pressing the key until generation of a shelf board impulsive sound, and selecting any of the plurality of types of delay characteristics; an extracting process for extracting a first delay time and a second delay time corresponding to a key-pressing strength generated by a key-pressing operation from within the delay characteristics selected in the selecting process; a string-striking sound generating process for generating the string-striking sound having a pitch depending on the key-pressing operation when the first delay time extracted by the extracting process has elapsed after pressing the key; a shelf board impulsive sound generating process for generating the shelf board impulsive sound when the second delay time extracted by the extracting process has elapsed after pressing the key; and a musical tone generating process for generating a musical tone having added thereto the string-striking sound generated by the string-striking sound generating process and the shelf board impulsive sound generated by the shelf board impulsive sound generating process. 