So your post says after following the first 2 suggestions (some other device is on the rails interfering with reading the CVs, that has been confirmed), good.
(you were finally communicating with the motor decoder)
Now you are trying to work on the sound decoder... right?
If the sound decoder is programmable, you have no way to isolate with the switch, does that make sense? You can have the motor decoder ONLY (by powering off the sound), but if the sound decoder is really a decoder, you CANNOT talk to it "only", there is no "sound only" switch position is there?
As Mark said, it appears the sound decoder is fed from the track, thus it MUST be a DCC decoder. You need to disconnect it, and connect it only to the programming track (and of course have the manual).
(this could be done externally with decoder lock I suspect, but I don't think that is a good option right now)
Greg