I found this txt file in the sound folder:
The list of songs replaced in Project M 3.5 is as follows:
A01 - Ground Theme (SMB) -> Metal Mario (Super Smash Bros.)
A17 - Mario Bros. -> Fountain of Dreams (Melee)
A20 - Mario Circuit -> Princess Peach's Castle (Melee)
B09 - 25m BGM -> Kongo Jungle (Super Smash Bros.)
C18 - The Hidden Village -> Hyrule Castle (Super Smash Bros.)
H06 - Victory Road -> Saffron City (Super Smash Bros.)
K07 - Porky's Theme -> Fourside (Melee)
R03 - Hanenbow Ambience -> Hanenbow by Garrett Williamson (feat. Adam Kirby and Audio Selah)
W01 - Princess Peach's Castle (Melee) -> Mushroom Kingdom 2 (Melee)
W03 - Jungle Japes (Melee) -> Hyrule Field Theme
W06 - Fountain of Dreams (Melee) -> Green Greens (Melee)
W07 - Green Greens (Melee) -> Gourmet Race (Super Smash Bros.)
W15 - Super Mario Bros. 3 (Melee) -> Yoshi's Story (Melee)
W26 - Kongo Jungle (Melee) -> Tal Tal Heights
X02 - Menu 1 -> Project M 3.5 Main Theme
All these files can be found in the sound folder along with the .txt file. Curiously enough, there's also a file called M18.brstm in the sound folder, but I have no idea what it is.