How can I amplitude modulate one channel with another ?

[steve]

If you want to also remove the carrier wave, try:

(sim (mult (aref s 1) -0.5)
   (mult (sum 0.5 (scale 0.5 (aref s 0))) (aref s 1)))

Just to be sure, I assume that this code:
-1- accepts two ordinary audio tracks (not necessarily for one of the them to be a pure sine wave and play the carrier)
-2- and its generated track will likely have a DC average mainly if the two tracks are identical (as in squaring; the first process to find RMS).

I have the impression, so I may be wrong, that this code multiplies each sample by its corresponding one on the other track (having the same timing) and normalize the multiplication result. So if the two tracks are similar, all new samples will be positive.

This is an old topic, and the code is using the old Nyquist syntax (deprecated).
In modern syntax the code would be:

(sim (mult (aref *track* 1) -0.5)
     (mult (sum 0.5 (scale 0.5 (aref *track* 0)))
           (aref *track* 1)))

• track is the selected audio that is passed by Audacity to Nyquist. In this case, a stereo track is required, so track is an array of 2 sounds.
• (aref track 0) is the sound from the left channel.
• (aref track 1) is the sound from the right channel.
• (mult (aref track 1) -0.5) multiplies the right channel sound by -0.5. In other words, it halves the amplitude and inverts it.
• (sum 0.5 (scale 0.5 (aref s 0))) multiplies the left channel by 0.5, and adds 0.5 to each sample value.
• sim is equivalent to sum. It adds sounds or numbers.

If the left and right channels are the same, then yes, all new samples will be positive.