mplayer/mencoder plays (undetected) mute for films/music with codec 0x50 & 0x55 (mp3lib)

[Alexis Scheuer]

When playing either a music or a film with mp3 audio, mplayer uses mp3lib but does not generate any sound, and mencoder transcodes the film into a mute one.
Forcing the audio codec ffmp3 does not change anything.
Other codecs do not work: mplayer "Cannot find codec for audio format" 0x50 (for films) or 0x55 (for musics) -> "Audio: no sound".

VLC handle correctly both the music or the film.

Both mplayer and mencoder are version 2:1.0~rc4.dfsg1+svn34540-1, from http://fr.archive.ubuntu.com/ubuntu/ precise/universe amd64 Packages.

ProblemType: Bug
DistroRelease: Ubuntu 12.04
Package: mplayer 2:1.0~rc4.dfsg1+svn34540-1
ProcVersionSignature: Ubuntu 3.2.0-27.43-generic 3.2.21
Uname: Linux 3.2.0-27-generic x86_64
NonfreeKernelModules: nvidia
ApportVersion: 2.0.1-0ubuntu11
Architecture: amd64
Date: Wed Jul 18 15:06:32 2012
ProcEnviron:
 TERM=xterm
 PATH=(custom, user)
 LANG=fr_FR.UTF-8
 SHELL=/bin/bash
SourcePackage: mplayer
UpgradeStatus: Upgraded to precise on 2012-06-12 (35 days ago)