see Sound

Registers

AUDV0 $19
AUDV1 $1A

Stella

These addresses write data into the audio volume registers which set the pull down impedance driving the audio output pads.
D3	D2	D1	D0	Audio Output Pull down current
0	0	0	0	No output current
0	0	0	1	lowest
0	0	1	0
...	...	...	...
1	1	1	0
1	1	1	1	highest