Created attachment 3223 [details]
minimal example to reproduce the issue

Consider the following:

    Mix_Music *m1 = Mix_LoadMUS("foo.mp3");
    Mix_Music *m2 = Mix_LoadMUS("foo.mp2");

    Mix_PlayMusic(m1, -1);
    // do things for N seconds
    Mix_PlayMusic(m2, -1);
    // do things for M seconds
    Mix_PlayMusic(m1, -1);

Note that I don't free neither m1 nor m2 in any case.

Here the problem: when I play m1 the second time, a few ms of the music from the point where it was when I stopped it the first time are played. Then it resets to the start and plays the whole music from there as expected.
It looks like a Mix_Music has a buffer it starts playing before doing any other thing.

Here is a Q/A on SO, actually only a Q so far - https://stackoverflow.com/questions/49836263/issue-reusing-a-mix-music-more-than-once
From the question above, you can get this link where you can find a short snippet to reproduce the issue (many thanks to the author) - https://gist.github.com/araml/7191ba537e497cc1ccd7e0199c65bdc6
I'm creating also an attachment with exactly the same code.

It happens with all the mp3 files I tried so far, so I don't think it depends on the file itself.
If you want a file that I used to reproduce the issue, here it is - https://ozzed.net/music/8-bit-empire.shtml#tune01