Hi Brigitte,
I understand your pain!
I generally make our socials posts, stories, reels etc in Canva and access their audio files to add music to our assets and they have been loading fine to Meta.
If I want to use a particular song that isnt on Canva, I find that if I use Meta on my phone it gives me more choice for adding music than it does through a computer, no idea why, its just another crazy Meta thing. I also sometimes post reels straight from my phone socials app, by switching to the College account and again it gives me more freedom with music too.
Hope this helps,
Kerry