audio in XVID file : should work but it doesn't ???

After heaving read all different comments about audio in this forum, and having done the necessary checks (and even installed the AC3 filter (0.63) which I think I do not need,
my XVID movie still refuses to play sound.
AVI codec gives me : "Audio : 107 Mb, 135 Kbps, 48000 Hz, 2 chan., 0x55 = MPEG Layer-3, Supported"
GSpot notes : "GSPOT has found that the Fraunhofer IIS MPEG Layer-3 Codec (Advanced) audio codec will decompress this audio format. Direct Show reported no erros, but failed to render the audio..."
What should I do ?

thanks

Spook