LG VX-8100

I am full of questions today. What's the difference b/t a "pmd" and "mid". All tones downloaded from MIDIringtones are "pmd". All other tones (VZW Tones, Sony Music Box, Modtones) are "mid". I noticed this when I set a tone from MIDIringtones as my alarm tone and then set my Master Volume to "Alarm Only". Well, when the alarm goes off, there is NO sound. It ONLY does that for the MIDI tones. All others (default phone ringers and other downloaded ringers from other apps) work fine. Anyone know the difference w/ the file type?

Thanks!