Music uses HTML5 Audio and Sound uses WebAudio. The difference is that HTML audio is an older standard that requires user interaction before playing and is pretty inflexible. The WebAudio backend is better and more powerful, it also allows for many manipulations like spatial sound.
@Muzan it is nothing against you but I often see factual wrong answers coming from you. Please do not share wrong answers. If you are not sure or are guessing, presenting an answer that is false will probably hurt more than not answering, in this case for example you are advising to use music for longer tracks where in fact for longer tracks too sound is better in most cases. If you want to verify some info before posting it, you can either try it out in GDevelop by yourself or if not possible like in this case search on the wiki for a page that backs your claim.