Apparently only Zackwell is having trouble with MP3 files with RMXP on Windows 10, so I think you're safe, Melana.
I did lots of research, and the only problems people have had with Windows 10 playing BGMs have been with Windows Media Player, the files were set to Read-Only after the upgrade, or they didn't update their Windows 10. Once they updated or changed the files from Read-Only, they were all fixed 100%, nobody ever had a problem like this. Literally 1 in a million...
Are you sure your computer is compatible with Windows 10, Zackwell? Because they did have some requirements, and it may have been bypassed automatically due to Microsoft forcing Windows 10 on everyone, and getting desperate to get people to switch... (Which would be wrong of them to force incompatible computers to switch, but they probably don't care...)
Have you contacted Microsoft about this issue? Because they could probably help you more
And, this may be a long shot, but... Have you tried to download a codec pack to see if that'll help? Because they tend to solve some problems with dumb codecs sometimes