Google may be able to help. I found the following information by typing "Kato GP-35".
http://www.katousa.com/consumers/HO-GP35-SD40-Circuit-Board-Conversion.html
Whether you can install sound or not is going to depend more on finding a location for the speaker than being concerned with the decoder.
I just purchased a Kato GP35 (H.O. Scale) and also ordered a Tsunami 828067.
Does anyone know of any tutorial that walks you thru how to install this decoder in the Kato? I have done a google search as well as looked at the Soundtraxx web site and haven't really found anything.
Michael