I moved to a new iPad (Pro 10.5"), never had this music issue on my iPad 2 before.
The new iPad came with iOS 12.2, and has the music issue. It's not completely gone, but only plays select pieces of music like the normal Egypt music, but as soon as a wave start, the music is gone. Lots of music pieces are missing. Updated the iPad to 15.2 because I thought the old iOS was causing the issue, but it's still exactly the same. This also proves that it's not an issue with a new iOS version, as it was the exact same with iOS 12.2 - I wonder what the problem is, and I really would love to get my music back!