You can control the music to any level, in three ways:

1. Edit the level using the the level editor. There's no problem with editing a pack made by someone else, so long as you're just doing it for yourself and don't intend to distribute the edited pack.

2. Open the level file using a text editor, find the "MUSIC" line (third line of the "Level info" section) and change this.

3. Open the level (by either method) to find the filename it points to for music, and save a file in your music folder with the identical filename.