A lot of the pre-installed apps have services that are interconnected. Deleting one can disable another as you've found out, unfortunately. System apps are not easily restorable like simply installing an .apk file. And once deleted, even a factory reset will not recover them. You will have to locate and flash a stock rom image to get them back. If the music player is all that's been affected, then maybe trying a different one from the market will allow you to play your media files.