After making sure it's not the volume pot, I suspect you have oxidation of the contacts on the two chips on the RailSounds board. I see the exact same symptoms regularly on the modular RailSounds boards. You need a PLCC extraction tool to pull the two chips up and then reseat them. That will clear the oxidation from the contacts for a few years. Do NOT try to remove the chips without the proper tool, you'll crack the socket and that's a much bigger issue.
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