Board Thread:Plants vs. Zombies/@comment-4775209-20150126194938/@comment-25230228-20160507235922

MO3 is a music file format developed by Ian Luck. MO3 files are tracker modules that contain samples encoded in the MP3 or Ogg Vorbis file formats, rather than straight PCM samples.[1] This results in a greatly reduced file size for the module, while maintaining almost identical audio quality. Lossless audio encoding is also supported, for samples that do not compress well with lossy encoding.

The format's name is a portemanteau of MOD and MP3.[2] Despite the name, five other tracker formats are supported besides MOD: Scream Tracker 3 (S3M), Fast Tracker 2 (XM), Impulse Tracker (IT), OpenMPT (MPTM) and MultiTracker (MTM).

PopCap's Bejeweled series, as well as other games like Zuma and Plants vs. Zombies use this format for music, using a module playback system that has the ability to quickly jump between and loop portions of the module which stores the music data, eliminating the need for numerous different module files.

from the wikipedia page https://en.wikipedia.com/wiki.MO3