I'm pretty sure it doesn't, nor does the LunaLua version. Also you don't necessarily have to convert it to MP3. It depends on what version of SMBX you're using. If it's SMBX 1.3.0.1 and less, then you can use MP3s, WAVs, and MIDIs. If it's SMBX 1.3.0.2 and more, including SMBX 1.4.2, then you can use
these music formats.