OK, so first and foremost mp3splt is quite old (last update 2014). Second thing, it relies on its own libmp3splt library and mp3splt-gtk package for the graphical user interface. All these three packages need to be added, starting with libmp3splt, for which I've already created a patch for submission in the repos https://dev.solus-project.com/T1334, if it gets accepted, because, after all, no other package will use this library except mp3splt. If the libmp3splt library gets accepted, we'll try to push mp3splt itself to the repo, but please note that it's only the command-line, and I have my doubts that mp3splt-gtk will build successfully on modern machines with today's GTK+ technologies.