ok, so these perhaps aren't exactly "secrets"...but they're definitely interesting tidbits anyway. Some of these are very well-known, but I bet that some of them might surprise you, even if you're quite familiar with Undertale music!
0:00 - Intro
0:04 - MEGALOVANIA
1:13 - Another Medium
2:03 - THE HOLY
2:38 - Ballchime.ogg
3:29 - Metal Crusher
3:57 - Man.ogg
4:26 - The Chase
5:31 - Your Best Friend
6:05 - When I Get Mad, I Dance Like This!
6:49 - Dating Start!
7:19 - Undertale
7:48 - Outro