Ok, that's a bit of a strange problem, that somehow crept into my system. It used to work fine.

Here is the problem as far as I can identify it. When I try to play certain video files with mplayer, there is no sound. As far as I can tell, it is only an issue with ac3 and dts sound tracks (using the ffmpeg decoder).

Mplayer says:

==========================================================================
Opening audio decoder: [ffmpeg] FFmpeg/libavcodec audio decoders
AUDIO: 48000 Hz, 6 ch, s16le, 1536.0 kbit/33.33% (ratio: 192000->576000)
Selected audio codec: [ffdca] afm: ffmpeg (FFmpeg DTS)
==========================================================================
[AO_ALSA] Playback open error: Device or resource busy
Failed to initialize audio driver 'alsa'
Could not open/initialize audio device -> no sound.
Audio: no sound

(similar with ac3 sound, but using the ffac3 audio codec).

Trying different audio output (-ao oss/pcm/sdl) doesn't fix the problem. The strange thing is that if I play these files directly with ffplay, they work fine.

mplayer sound with mp3/ogg is fine

My alsa configuration is standard (no /etc/asound.conf or ~/.asound*)

OS: Linux Gentoo
Mplayer: 1.0_rc4_p20100213 (SVN-r30554-4.3.4)
FFMpeg: 0.5_p20601-r1 (SVN-r20601)

Any other information I can provide?