I'm not sure about JMRI, but with SoundLoader you can listen to the individual segments of a sound scheme. I don't recall exactly how it's done, and I'm not near the layout computer, but I have done it. I do recall that it involved listening to the sound scheme file itself, downloaded to the computer, and not the scheme actually installed in the decoder.
But maybe you're thinking of the "neutral" state that the PR3 sets Digitrax sound decoders into? That allows you to "run" the loco with the motor drive turned off. Check the JMRI Yahoo! group's archives. There was some discussion about this not too long ago.
EDIT: Found more info on the "neutral" state: Look on page 29 of the "Soundloaderhelp.pdf" document that's included with the zip file available here.
Sorry, no can do.
You have to be on the main to hear any changes to the sounds when using JMRI.
Here is what I do. After I finish a sound decoder install, I put it on the programming track, enter the roster information, then read all the decoder settings and save them. Next I change the address of the decoder to match the loco number and save the file again. Then I remove the loco from the programming track and put it on the main. Close the JMRI decoder pro from using the programming track, and open the 'Program on Main' section. Choose the loco from the roster and open the advanced programmer.
Now I do all other programming on the main with decoder pro and test each thing as I go along, and save the file after I am satisfied with each change.
Elmer.
The above is my opinion, from an active and experienced Model Railroader in N scale and HO since 1961.
(Modeling Freelance, Eastern US, HO scale, in 1962, with NCE DCC for locomotive control and a stand alone LocoNet for block detection and signals.) http://waynes-trains.com/ at home, and N scale at the Club.
I recently set up a Digitrax PR3 with a programming track connected to my computer. I'm sure I read that using JMRI Decoder Pro and programming a sound decoder on a programming track, that there was a way to "test" each sound - actually play / hear it - while it's still on the programming track.
As usual, I'm sure I read this somewhere but can't remember just where. I've searched the JMRI info and can't find anything. Is it maybe some other system (Sound Loader??) that I'm confusing with Decoder Pro??