Tried several things and still no recovery. Followed all the instructions and tried the fixwav fix and the Audiohack fix. Tried chkdsk /f and chkdsk /r. Tried the free versions of RescuePro Deluxe, Recuva, Klennet Recovery (which has replaced ZAR), and Data Rescue.
The AudioHack fix creates 2 files from one of the zero byte files, both are only 1k in size. Fixwav message attached below.
Importing the zero byte files into Audacity as raw data does nothing.
I did find that there's a 1.38GB file named FILE0000.CHK on the card. I made a copy and renamed it to .WAV and pulled it into Audacity as Raw Data and it's a combination of white noise and music. As advised I imported it with a different starting offset, 1 byte at a time up to 6 bytes. Still white noise and music, but at some of the starting offsets it loaded into Audacity much faster.
The fact there's a 1.38GB .CHK file seems promising, but is it? The message I got from trying fixwav on that looked promising (see attached).
One thing I have not yet done is remove the 7.78GB worth of good WAV files from the SD card. They're backed up elsewhere but I have hesitated to make any changes on the card itself.
Like I mentioned earlier, the right amount of space to accommodate the three zero byte files (if they were each about 1.3GB or so as I'd expect) is there on the card. So I'm not giving up but am at a loss as to what to try next.
I've also attached some screenshots of messages from trying some of this.