So, the main theme is unique in this game. Whenever you exit a match, or enter the menu again for whatever reason, the song has multiple places in the song it can continue from. My question, is whether or not this can be replicated to additional songs? If so, there's a good amount I would like to add in.
Yes, this is possible. In Sm4shMusic, select the song you want to set custom start points for under "BGM Management". Next, click the button that says "Adv." next to the name of the .nus3bank used for the Song ID. A new window labelled "BGM properties for (insert .nus3bank name here) will appear. On the right side of the window, four values labelled "Menu Checkpoint 1-4" are present. They will be set to -1 by default, but can easily be changed to whatever you please. In these boxes, replace the -1s with the points where you want the song to start in milliseconds. After that, close the window, save your changes in Sm4shMusic, and prepare your modpack for play as normal.