Probably not a good idea to mix them. I know that if you ship files to a duplicating house they want either all mp3's or all wav's. And, of course, older CD players won't play mp3's at all.

I was actually thinking of a CD-ROM, which does let you mix file types. (It's possible to make double-session audio/data CD/CD-ROM disks, but it's tricky and they seem to cause problems in some players. Like the one in my car.)