Hello and Welcome to the world of RailPro!
I too was all DC and chose RailPro. If you want sound, you'll need a LM-3S regardless. You cannot use the sounds that are currently on your sound equipped locos, but you can reuse the speakers. You'll need to load the required sounds onto your LM-3S module using the RailPro Assitant. The new ULT sound files are fantastic.
I personally don't have any Broadway Ltd locos, but I'm sure someone on the group has done a conversion and can help you further.
Don't be afraid to ask any questions - there is a TON of knowledge in this group!!