this is a proprietary format developed by Microsoft. Users have noticed low error resilience: sometimes insignificant damage to the file can cause the file not to play. Besides this, the closed format hampers its proliferation outside of Windows platforms.
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.