Yup, AmyGrrl is right on the money. I've made accurate rips from the original, and still the MP3s are slightly out of sync. It's not horribly out of sync, but it's off just enough to annoy the hell out of me
I think it's the Fraunhofer MP3 timing bugs, but who knows.
There are two Sega CD games I know of that people should really avoid ISO+MP3s versions at all costs, and just wait to find disc images (bin/cue) for. Lunar is one, and Snatcher is the other.
If anyone's using XP, don't forget you can compress the isos/bins (via compressed folders) to save some space, and mount the images using something like Daemon tools.