import random


def augment_volume(wav, rate_lower=0.7, rate_upper=1.3):
    """
    Increase or decrease a waveform's volume by a randomly selected rate.
    :param wav: a waveform.
    :param rate_lower: lower bound of rate
    :param rate_upper: upper bound of rate
    :return: 
    """
    return wav * random.uniform(rate_lower, rate_upper)