Changing volume when audio is disabled was a feature request (github
issue #215), and was introduced with commit 327a779.
But trying to fix github issue #280 (volume is not correct in no-audio
mode, and if audio is re-enabled, the volume set in no-audio mode isn't
set), I concluded that it's not worth the trouble and the current
implementation is questionable all around. (For example, you can't
change the real volume in no-audio mode, even if the AO is open - this
could happen with gapless audio.) It's hard to get right, and the
current mixer code is already hilariously overcomplicated. (Virtually
all of mixer.c is an amalgamation of various obscure corner cases.)
So just remove this feature again.
Note that "options/volume" and "options/mute" still can be used in
idle mode to adjust the volume used next time, though these properties
can't be used during playback and thus not in audio-only mode.
Querying the volume still "works" in audio-only mode, though it can
return bogus values.
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