There is only one file type that uses the .W64 extension:
Sony Wave64 Audio File
Audio container file created in Sony's Wave64 format, which is supported by various Sony audio editing programs; useful for audio editing because it supports arbitrary sampling frequencies, channels, and bit depths.
The W64 format was designed to surpass the 4 GB limitation of Microsoft's 32-bit RIFF/WAV specification. W64 files are 64-bit and allow for file sizes greater than 4 GB. This enables audio files to have a much longer duration.
NOTE: The Wave64 format was developed by Sonic Foundry but now is maintained by Sony. The official name of the format is "Sony Picture Digital Wave 64."
Updated: December 1, 2016