I mean no disrespect and not trying to nitpick, but you do not (and should not) include your .def file in your .zip file. I compared it line for line. Your .def is 100% identical to the current games' Sound_Music.def . The only exception is a couple of //comment lines, which means nothing.
You said your .def changes the play order. Well it appears that they have adopted your order if that is the case (the 'what plays where'). Because they are identical.
In fact, just yesterday was a new update to Sound_Music.def which fixed the 'Old Rachival' music to start playing again (which has been broken for a VERY long time, Years maybe?):
string name = "ancient_ruins_mus" was changed string name = "mus_ancient_ruins"
So as of two days ago, your .def is now out of synch. By simply removing the .def from your .zip, this would allow it to play correctly (and use the new changes). I heard there are some new sound mods in the pipeline and this may or may not further change the Sound_Music.def in the future; which would then cause your .def to be even more obsolete. Your mod plays just fine without the .def. Trust me, I've tested it quite a long time with different scenarios. So I'm just saying by removing the .def you will allow your mod to keep working but also use any latest game updates. Your MP3's will always override the current .mp3 files even without the .def. Just my 2 cents. But I really like your mod, I've added it to my
mirror to be forever downloadable and available (without the .def file in the .zip), which I feel is most forward compatible
By keeping your .def in the .zip file, you might override and prevent future game patches from working correctly (ie the recent Old Rachival fix).