Troubleshooting audio problems

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symptom:
- no audio in some applications.
- pulseaudio only shows a dummy audio output channel.

$ pulseaudio -k
$ pulseaudio -D
E: [pulseaudio] main.c: Daemon startup failed.

Check if there is any application blocking the output sound card:

$ fuser -v /dev/snd/pcm*
                     USER        PID ACCESS COMMAND
/dev/snd/pcmC0D0c:   overshoot   4454 F...m Application
/dev/snd/pcmC2D0c:   overshoot  12878 F...m pulseaudio

Quit or kill that application and try again.