Volume control for music and sound.

[supercom32]

Possible enhancement identified using the latest version of Dragon Basic under Linux (Commit ID: d2ce042)

When working with Dragon Basic, I noted that for music and sound there is no way to control the volume of Direct Sound channels. That means users can't fade in/out music as gameplay/scenes change and the only recourse is to abruptly stop it. Looking at the GBA registers for sound, it sounds like "REG_SOUNDCNT_H" only allows for the following options:

Output sound ratio for chan.1-4 (0=25%,1=50%,2=100%)
Direct sound A output ratio (0=50%, 1=100%)
Direct sound B output ratio (0=50%, 1=100%)
Unused
Direct sound A to right output
Direct sound A to left output
Direct sound A Sampling rate timer (timer 0 or 1)
Direct sound A FIFO reset
Direct sound B to right output
Direct sound B to left output
Direct sound B Sampling rate timer (timer 0 or 1)
Direct sound B FIFO reset

Seems like unless some extra processing is involved in software, the hardware can only set the output volume to 50% or 100%. I know some GBA module/tracker libraries can manipulate volume, but that might all be done in software. This is probably a hard enhancement to fill, but I thought I'd mention it for informational and tracking purposes anyway.