Hello,
I also don't like the menu music from Winds of War expansion. I managed to replace it by overwriting two files in the GOG Games\HoMM 4 Complete\Data folder. You can download the files from:
storm_override.h4r
x2_override.h4r

Warning: Before you replace the original files in the game folder make a copy of them and store somewhere. It's in case if the replacement files won't work.

Tip: By replacing both of the files the menu music is going to be the base version of the game. But by replacing just the x2_override.h4r file it is going to be music from The Gathering Storm expansion.

It would be much better to leave the game files as they are, and pack the main menu theme into a new h4r file. For WoW, you would call the h4r file new.h4r. For Equilibris, you would call the file new_mod.h4r. Simply place the file in your H4 data directory.

Here is a link to the H4 resource editor thread, which can pull the menu sound file and then pack it into a new h4r file.

If I remember when I get home, I'll create the files and post them here for anyone else that wants the original music instead of the WoW expansion music.