[RM2K3] USING MP3

How to use MP3? I copy the exact file from Alter Alia (RM2k3) just to test it out.

I could hear the music from the test (inside the RM2k3), but when I test playing it (F9) or even double-clicking the RPG_RT.exe, the music couldn't be heard.

So, what I have to do to use MP3?
(because I have a music cost 900kb in WAV compared to 300+kb in MP3)