From Original War Support Wiki
Tutorials -> Adding Music
Changing the menu's background music
In order to change the menu's background music, you have to convert your music file into a .wav format (WAVE; PCM; use an encoder).
The file has to be placed in /%MODDIR%/sound/hudba/ and added to mods/%MODDIR%.ini
To add your music file to a .ini file, simply open the .ini file and add the line below:
Mod_Music = "your_music_file"