The game IS old school and so it's its music, using much simpler midi music in all the themes. Like Heroes III, every faction in the game has their own theme but all these themes are really good as well. Where the game doesn't decide to go with midi it actually uses opera singing. Opera singing in a game this old!? It's pretty impressive in my opinion, not to mention that the singing actually makes the music really enjoyable and even more "epic" than it would be without. The game combines the midi sounds, singing and some real instruments as well in it's songs.
Though the confusing part is, the music got changed around in different versions of the game, so I am not FULLY sure which are the original versions of the songs.
Here is a few of my favourite themes
Sorceress Town Theme
Warlock Town Theme
Necromancer Town Theme
Barbarian Town theme