ProtoSound2 - sound cutting out - barely audible

Does this loco have a 9 volt backup battery? The speakers in Protosounds 5 volt locos have an ongoing speaker problem, the speaker magnet material deteriorates, causing the problem. Usually the speaker has the magnet crumbling, but not always. A speaker is cheap if not the problem. Also, if the battery is low, the sound output will drop after startup, until the battery is fully charged. This most often is apparent when using a BCR. Once the BCR has a charge, the sounds come up to the set volume. 

Should be 3V model.  This can be a bad battery.  Model is from 2007.  I would replace battery, but unplug battery and start it up to see if sounds remain.  If so, replace battery.  Otherwise could be hardware fault.  Also inspect speaker.  G

I'd be looking at the speaker and the volume pot first.  You can bypass the volume pot pretty simply to see if that's the issue.

Not me I look at battery first.  By the way if running in DCS volume pot does not matter.  That is conventional only.  G

Good point about the volume pot, and you had already mentioned the battery.  So it's battery, speaker, wiring, board?  Let's hope he doesn't get to the board.

One thing that I've seen once is a corrupt sound file, maybe try reloading that if all else fails?