are you using gmplayer or the non-gui version?
please also attach the full command line output.
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mplayer doesn't use 'pulse' audio output by default
https://bugs.launchpad.net/bugs/550517
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Public bug reported:
Binary package hint: mplayer
mplayer doesn't use native pulseaudio by default. You have to specify
the '-ao pulse' option on the command line.
It seems to use oss, or bypass the pulseaudio alsa redirector. This can
mean sound comes out of a different sound card than the one