It's the newest version, galaxy s8 plus, at&t, stock camera, and it records video no problem, but if I have music playing and want to record with the music in the background I can't, bc it just automatically stops the music when I start to record
Stopping the music, or any other sound from the phone, makes sense from the photography perspective. You want to remove all sound creation from the phone because you want to hear the sound from the subject where the phone is usually not the desired source of the sound.
Most likely, the camera app is turning off the sound app when you go into movie mode. Assuming this is the case, you could try other camera apps and see if any of them will work for you. I would not get my hopes up though.