How do you get amplitude modulation (tremolo) based on a sample?

[Trebor]

The only problem is that this creates two oscillations per wave (one for each polarity).
I only one want one oscillation per wave, like this:

Use Audacity’s pitch-shift to drop the frequency of the modulator audio down by an octave, before multiplying.

One way to achieve that envelope shape is to full-wave rectify the modulator audio then use the multiply code …