Joel - I've used some of the sounds found on those websites for my own use, with the owner's permission. I'm not sure that's necessary for your own use, but I like to be safe. Some of them are good, some okay, some not very useful. A few of the authors have very nice recordings, but many are so-so and really don't sound great on our miniature systems. They are great to experiment with though and get a feel for how to edit sounds.
It does not matter what format the original source recording was in. It DOES matter though that recording you load onto Railpro be 44.1kHz, 16-bit, mono WAV. If the source recording was recorded at different settings (most often the sample rate is different) it can be easily changed using programs such as Audacity, which is free.
What really makes a difference is good recordings from Railpro users. It's not too expensive to get the equipment to make decent recordings. The hard part is having access to railroad equipment to record. Most people have better connections than I do and so should at least be able to record something. Even simple things like a horn or bell, or air blasts. Anything is useful.
Bill - There should be a search option on TrainSim that allows you to sort by file type. Search for MSTS sounds and what you are looking for. There is a decent selection, but again not all recordings are created the same. And watch for copyright issues.
-Kevin