Looking for the music for Metal Slug 1-6, Metal Slug X, & Metal Slug XX. Any help is always appreciated.

You're looking for the Box. Shadowhaxor has it, as does Metal Slug DB. They have Advance, 1, 2, 3, 4, 5, 6, 7, X, and 3-D, but not XX, 1st Mission, or 2nd Mission.