I know this is probably more of a GQ, but it’ll probably get chatty, so…
is there? 
I haven’t been able to find one on palm.com or on ebay…
Unless there was a dramatic increase in drive space in the newer Palms, I would say no, since the drive space on a Palm is so small you could really only get one or two MP#'s on there.
My understanding was that there was a clip-on attachment that would store and play the mp3s… I’m fairly certain they have it for the higher-end palms… although I may be mistaken.
There’s a few MP3 players for the Handspring Visor, but those of course use the springboard technology Palm doesn’t have. The two available options are also quite overpriced, IMHO.
I don’t know of any solutions for the Palm Vx, and a quick search didn’t reveal any either.