MOD is a sound module format used to store music tracks and uses a .mod filename extension. It features a collection of sample instruments, and the samples are assigned to varying music tracks. The sound samples are opened, depending on score notes. Today, MOD can be opened using various audio tracking programs.
M4B is an audio filename extension for AAC format, which is developed by Apple Inc. It is mainly used on podcast and audiobook files. Typically, M4B files contain audio and other metadata like images, hyperlinks, and chapter markers. M4B files are DRM copy-protected and can be opened on Apple iTunes and QuickTime Player.