It's possible! See, all of the music files in this game are stored as seperate OGG files. If you've ever worked with Audacity, you'll know what an OGG file is.

They're located under the folder 'OGG'.

Okay, I see a bunch of random files. What do I do?

Well first, you make a backup of the file that you wanna replace (IE, if you're replacing the first stage's music, make a backup of st00.ogg). Lolwut?
You do that so that you can always change it back.

Now, go into Audacity, and import the MP3 that you wanna use. Export as on OGG, and replace st00.ogg or whatever with that file. Make sure the name is the EXACT same. Now go play MMX8 and enjoy the music.

For example, I replaced the intro stage's music with the green forest theme from Sonic Adventure 2.

@ Mods: Sticky pl0x :p