I found a way how to add any music file into H4. I'm not sure about link posting but here is my light "mod" which replaces grass theme with "Wandering" theme and main menu theme with my so-called "remix" (i just paste together main menu themes from all three versions of game and fix differences in volume level - starts of course with Gathering Storm theme).
http://depositfiles.com/files/4g3h3xvvx - just put this into Data folder.
Step 1
So if you have any mp3 file (CBR 128 kbit/s, joint stereo, ms stereo, 2ch, 44,1 kHz). Open it into hex editor. Get the full file size by pressing Ctrl+A in hex format (e.g. my sound.main_menu.mp3 has 3E1AE0). Write it somewhere in reverse 4 byte format (e.g. 3E1AE0 -> 00 E0 1A 3E).
Step 2
Then open your file in some kind of audio editor (im using adobe audition) and write down samples number (in my example - 11218176 samples). Then multiply this value on 4 (e.g. 11218176 * 4 = 44872704). Convert it into hex (44872704 dec = 2ACB400 dec) and finally write it in reverse 4 bytes hex (2ACB400 -> 00 B4 AC 02)
Step 3
Return into hex editor and add such 13 bytes into beginning of file (header).
01 00 00 10 02 44 AC 00 00 00 B4 AC 02 01 00 E0 1A 3E 00
01 00 - ??
00 10 - 16-bit (per sample)
02 - stereo
44 AC 00 00 - 44100 Hz (Sampling rate)
00 B4 AC 02 - sample count*4 which we get in step 2
01 00 - ??
E0 1A 3E 00 - exact mp3 size which we get in step 1