Find the time lag for a wave
The time lag is equal to the index difference times the sample period.  The sample period is:
$$
T_s = \frac{1}{f_s} = \frac{1}{16000} = 62.5\mu s
$$
Thus, the time lag from the first peak to the second is $(12 - 2) * T_s = 625\mu s$.  The time lag from the second peak to the third is $(25 - 12) * T_s = 812.5\mu s$.