There is only one file type that uses the .NVF extension:
Creative Labs NVF Audio File
Voice audio file recorded using some of Creative Labs' MP3 players; stores audio in Creative's proprietary NOMAD II Voice Format (NVF), which uses G.721 mono Adaptive Differential Pulse-Code Modulation (ADPCM); used for storing voice recordings, such as personal memos, on Creative devices.
NVF files can be played in iTunes, or using Creative's NOMAD II Manager software.
Updated: May 13, 2015