CVU file is a CVSD Unfiltered Audio. Continuously Variable Slope Delta modulation (unfiltered) is an alternative handler for CVSD that is unfiltered but can be used with any bit-rate.
M4A is an audio file format that is very similar to MP4. It is a proprietary file format of Apple. iTunes store contains the audios as M4A format. It uses MPEG-4 codec to contain audio files. One of the basic differences between M4A and MP4 is M4A does not contain videos like MP4.