I just tried doing the same.
Picked a random .mp3, converted it to .wav (WAVE(PCM)) making sure I match the original (main_menu_tune, 44100Hz, 1411kbps, 16 bit, Stereo)
Put it back in the sounds.zip (C:\Program Files\Rigs of Rods\resources\sounds.zip), enabled "main menu music", restarted, and it works.
It did not work when I got things super wrong (48000Hz, 1152, 24bit, Mono). So I guess you really have to match the parameters.