Thanks for the reply. I didn't think it was too likely, but worth asking.
Thanks too for the conversion suggestions. What little I've found on the format suggests it uses a proprietary variant of FLAC, which I suppose implies it is lossless. I know it supports some tags to some extent.
The current version of Sony's Sound Organizer software can play dvf, but only files created after somewhere around 2012. For older files, I need to use the program that preceded Sound Organizer, "Digital Voice Editor," which does not run on Windows 10, possibly nothing newer than Windows 7. As it turns out, my local network is running on a Windows 7 machine, with Digital Voice Editor installed, and that opens the files that the newer one does not, and can convert them in batches to mp3 or wav.
Another reason to keep my Windows 7 machine chugging away.