Make the two audio tracks into a stereo-pair, then apply this code in Nyquist prompt to multiply them …
(mult (aref *track* 0)(aref *track* 1))
The right channel will be amplitude-modulated by the left.
Using the identical track for both channels will probably sound horrible.
IMO need to low-pass-filter, or pitch-shift-down, the left version if it’s a musical-instrument or voice.