The tonedec program is designed to decode some WAV files created with 
the ComX35 DSAVE command. The only byte values read are '0-9', 'A-F', 
'%', ':' and "\n". 

In the ComX35 string format, the string contents is proceeded with a
"%" and terminated with a "\n". The basic program on the ComX35 side,
uses a ":" to separate address from data. data is saved in numbered 
blocks, but this doesn't seem to interfere with the hex string data.

The expected wav-file format is mono, 22050 Hz, 8 bits, unsigned. 
The resulting file needs some minor editing, due to incomplete 
(=dumb filtering), but this is quite easy.
