Sound disappeared

I'd suspect the speaker first, and a blown amp second.  If the speaker has shorted, it could have damaged the amp.

First step would be to check and/or replace the speaker.  If a new speaker doesn't do it, my bet is on the amplifier, those can be repaired without replacing the board.

Took the cab off this afternoon to look for something obvious.  Didn't see anything but sound came back when I put the chassis on the track. I then removed the fuel tank screws and moved the tank/speaker back and forth and the sound went in and out a few times then went out permanently.  I think it's the speaker so I unsoldered it and briefly hit it with a AAA battery and nothing.  Plan on stopping by MTH tomorrow to see if they have a replacement.

Speakers can fail. Best to catch it before it damages the audio amp on the processor board.  G

Have you checked the volume control(s)? Wandering thumb incident.

Check closely at the speaker connections.  I've seen two of them recently where the small braid that goes from the contact to the speaker coil through the cone were touching the frame of the speaker.  One survived, the other board set is still in intensive care and it doesn't look good.

I would also inspect the wires going through the chassis for wear, a chafed wire can short out and blow the amp.

Or more than the amp!  I have one board set that was thoroughly toasted by a short to the frame of a speaker lead!  I had another recently that all it needed was the audio amp chip.

Update: Picked up a new speaker at MTH on Thursday.  They had an exact match.  Installed it and everything is back to normal.  One note however.... the speaker wire leads that pass through the chassis into the fuel tank were extreemly short and might have been pulling on the connector tabs on the speaker which could have put tension on the voice coil lead wires.  I pulled another inch of wire into the fuel tank to eliminate this possibility in the future.  I wanted to post this followup in case anyone else had a similar problem.  Many thanks to those who posted tips.