I managed to fix it finally. You can use either of the following ways:
(1) In gmplayer > Preferences > Codecs and demuxes
Change Audio Codec to "FFMpeg/libavcodec"
(incidentally, the mplayer site docs list this codec as *the*
fastest codec for decoding Mpeg4/DivX etc
(2) In ~/.mplayer/config, add the following line:
afm = ffmpeg
(3) If you're are the command-line types, you can run mplayer as follows:
mplayer -afm ffmpeg
It was just a problem of selecting another codec
- Nocturne