No. It won't be a codec problem. If the video plays then you have the codec.
The sound problem may be related to Pulse Audio, but that is just a guess.
PA in 9.10 does not always switch well between different applications. If
you were using sound in two applications it may not work in the other. I
don't like PA and prefer ALSA for that reason, but I am told that this is
fixed with 10.04. I don't know because I removed PA.