As you might understand, this is quite easy. 1.Go to edit mode 2. Type this two lines of code:
INPUT "What music";MSC BGMPLAY MSC
3. Thats all! For a list of the ingame music, go to : http://petitcomputer.wikia.com/wiki/Preset_Music
Of course you can also make your own music. Hope this was usefull!
I didn't had an account when I made this article, but it is made by Cookiebas.