Only where it is provided, I remember you can add music that will play in the arena (Battle-Themes), but for example, you cannot add new music to the town menu, only replace it (Town-Themes).
Well, I was able to replace it. Terrain sand (terrain snow) and Town-Academy (Town-Tower).
I am really annoyed by the mages in the 5th game and their music (subjectively).
But it would be great to mix it with the originals from the 3rd game.
In principle, if we reason logically, it seems that Battle tracks are divided. At least for 1 race (faction), I see 2 options there, since there are 2 of them, it means they are taken either in turn or randomly. There, I think options 2, 3, 4 will easily fit.
And most likely, each race is tailored to itself. And this means that if you want to make a battle theme from the 3rd game for all races, you will have to duplicate it several times and rename it for each race.