I saw in SEGASATURN MAGAZINE, there's a part of a walkthrough listing some of the hidden tunes that play when you enter the appropriate initials. Titles are listed, so that can help with the occasional game that hasn't been released to album.

The 3rd batch is all the ones selected by developers, but .KK is the most interesting since it's an original song called Theme of Kouchi. Takenobu Mitsuyoshi, Takayuki Nakamura and Kazuhiko Kouchi made the jingles, so that's probably a composition by Kouchi.