Considering how AWFUL it was to mod in back in the day - and by “awful”, I mean “impossible”.
Just expose the playlist txt file (after all, BGM mechanisms in Warcraft 3 aren’t exactly sophisticated, just tracks in succession), with guidelines for supported formats and eventually support in case of custom races for maps who want to add their own music. This includes victory-defeat music, naturally. As of now, supported formats are MP3, WAV and FLAC (apparently all in-game sounds are flac, which I can respect!)
Further specified this as SOUND modding in general, rather than music modding, since in the original game it was impossible to mod soundsets too.
As an alternative to the above solution, another one would be to allow, for maps, the “asset import” function to act as an override for files inside the CASC archives - if you put a file with the same name as part of that, it takes precedence over the archived one.
If not, give us tools that allow for repacking or injection of CASC files, or an actual “override” folder in the game data folder working similarly to the Neverwinter Nights one.