You need to specify exactly what PS/3 locomotive you're interested in. The sound files are typically posted for each individual locomotive on the web page. Click on the icon and you'll get the sound file package in a ZIP file. Unzip the outer layer and you'll end up with the chain files in a ZIP file and the sound file in an MTH file.
I want to replace the station announcements on a Union Pacific City of Las Vegas Aerotrain with those of a PRR Aerotrain.
Btw, is there a way to hear all the announcements, crew talk, etc. in a file before you actually install that file? Any program or app that can be used to preview sound files?