Okay. This happened just this week on my layout. a PS2 engine I had sitting on one of my sidings for months, inactive. Couple days ago I decided to run it. It came up "not on track", yet started moving to my mainline. Suddenly the sound was quiet, yet could still hear almost a whisper the bell and whistle when buttons pushed. Was thinking a reset, bad speaker, etc. but decided to try something else.
I pulled engine off track, cleaned pickups and wheels and the track siding were it was sitting, (boy was it dirty). Put engine back on siding, started her up, (this time did not get the "not on track" message). Sound was back loud and robust.
Might want to consider cleaning wheels, pickups, and track first.