Using GrAnd release 148 on a Canon S3,
when I request IMG_ as RAW file prefix, I get SND_.
My other settings are:
Save RAW yes
Noise reduction Auto
Only first RAW in series yes
RAW file in dir with jpeg yes
RAW file extension .WAV
My rationale for using IMG_ as prefix is to sort the raw file just next to its basic jpeg so I can easily erase them together when necessary.
Thanks