I have been capturing steam whistle and bell sounds from YouTube videos then editing in Audacity to RailPro compatible .WAV files. My sources have been videos of the East Broad Top Railroad. So far I've captured and converted whistles from EBT 12, 14, 15 and 16. They can be heard in the following two videos...
For each whistle I've created a Crossing, Forward, Reverse and Stop signal. For a few I did my best to create a loop files as well. If there is interest, I'll share the .WAV files here. You would need to import them into the RailPro Assistant since I have not been able to find a way to export sounds from the Assistant once created.