Check the original Hardware Manual, p. 5-18, section 5.3.3 Modulating
Sound. Bits in the ADKCON register: ATVOL3 through ATVOL0. Setting the
ATVOLn bit makes channel n modulate channel n+1 (ATVOL3 just shuts off
channel 3 audio output) volume. In this mode, for each 16-bit word of data
fetched for channel n, only the low 7 bits, V0 through V6, are used; the
rest are ignored. Each time the period register of the modulator channel
times out, a new volume is fetched and stuffed into the volume register of
the adjacent channel. It gets wierder if you also want to modulate the
frequency, but that's a different story…