I had this problem about a week ago. Switching between SD and HD channels along with switching and playing SD and HD recordings for a few seconds fixed the problem. Unfortunately I cannot remember exactly what bit fixed it, but I’ll pay more attention next time.
It happened again two days ago; however no amount of switching or powering off helped. I noticed that sound was not available on SD only transmitted channels, but available on HD and SD channels transmitted alongside HD (i.e. SD on the same HD Mux).
I checked the fvp4000t volume and this was set at minimum instead of maximum. Obviously the fvp4000t shouldn’t have lost this setting.
Next time it happens, I’ll check the fvp4000t volume first, switch between SD/HD channels, check stereo/dolby-digital in the audio general settings before trying powering off or resetting etc.