Ian,
I an still guessing that you are dealing with an id3 tag or header corruption issue. Not being familiar with Winamp and the degree to which it accommodates errors, I can only suggest that being able to play the tracks in question via a Samba share using that client may not be diagnostic enough.
What happens when you mount the share and attempt to play the tracks through iTunes instead?
If you would be willing to send me a tarball containing one or more of the files in question, I would be happy to try to help diagnose the issue.
Herman
[Herman.May@earthling.net]