I had a similar problem with an FP45 and True Blast sound - sound was very intermittent would sometimes work - sometime not - be very low - just overall inconsistent at best but mostly not working - bell or horn.  I had it running on a Fastrack system from a polar express set - CW-80 - thought it was something with the transformer (based on everything that I've read) . 

I Expanded the set recently and replaced the CW-80 with a z-4000 - same problem.  Sound would be inconsistent at best or not work at all.  This post inspired me to have a look - in my case the potentiometer was straight up - but something else was off with it - I remember having a stereo amp that would get static when adjusting the volume - dust or dirt between the contacts  - I adjusted it up and down briskly a few times tested it on my new bench CW-80 and it started working "as designed" - so dirt or poor contacts inside the potentiometer was causing the flakiness there were no soldering problems that I could detect prodding around the connections - I did reverse the board moving the potentiometer away from the speaker - all of the leads were long enough - this was to get better access to the volume control without torquing  anything it back together and it appears to be working for now. 

Thank you for posting your info - you've now fixed 3 Locomotives

-Phil