outgoing.go
func (oa *OutgoingAudio) SetDuration(to int) *OutgoingAudio {
	oa.Duration = to
	return oa
}
func (oa *OutgoingAudio) SetDuration(to int) *OutgoingAudio {
	oa.Duration = to
	return oa
}
// SetDuration sets a duration for the audio file (optional).