After an upgrade to Debian testing this weekend (jan 23, 2010), audio
playback on my desktop GNOME machine was left in a compromised state.
None of the following worked following the upgrade (they worked before):
By `not work' here, I mean that the application seems to think that it
is playing the sound-file (there is no error message), but no data is
actually sent to ALSA (as far as I can see), and nothing is therefore
audible. Watching the levels in the mixer (no activity at all)
suggests that no data is being directed to the sound-card.
It's not the case that ALSA is broken. The sound-card is properly
detected and registered in /proc/asound/cards; alsamixer works fine,
and the low-level command line utilities like aplay and mpg321 work
beautifully. Xine, mplayer, and its derivatives (smplayer, kmplayer,
gnome-mplayer) work very well.
With VLC and audacious(2), I was able to fix the problem by re-setting
their Output plugin from `Default' (which had worked before) to Alsa
or (in the case of VLC) to Pulseaudio or Jack.
I suspect that what these have in common is that they use gstreamer as
their backend, suggesting that that is where the problem ultimately
lies. Gstreamer is installed on this system: