Is Classic Amiga & DOS Tracker Music Pack free?

Yes. Classic Amiga & DOS Tracker Music Pack is released under Open Source (GPL-3.0) and costs nothing. Neither it nor Mrboom has any paid content.

What do I need to run Classic Amiga & DOS Tracker Music Pack?

Mrboom, which is itself free. Classic Amiga & DOS Tracker Music Pack is an add-on and will not run standalone.

The PlayBound Launcher installs it in one click from Mrboom's mods list. You can also install it by hand — full steps are on this page.

Roughly 25 MB, on top of the base Mrboom installation.

Yes. The source is published at github.com/Javanaise/mrboom-libretro under Open Source (GPL-3.0).

Classic Amiga & DOS Tracker Music Pack — Free Mr. Boom Mod · PlayBound