Hello and first of all, thank you for making what is possibly my favorite iOS app! :-)
So, my suggestion is probably self-explanatory, but my idea is that if you have the Play/Prev/Next controls on the lock screen or the Control Center, these buttons actually change to the next subsong instead of the next file.
The way I use Modizer, I usually have a game's music rip open and frequently skip some (sub)songs. The prev/next buttons aren't useful for me, so I have to unlock the device every time and do that in the app itself. I almost never want to skip directly to the next file.
Personally, I think Prev/Next should refer to subsongs by default, but it would be okay if there was an option somewhere.