After trying many more ideas to fix this bug, I think I may have fixed it!!! (at least for my circumstances)
I discovered my singers were all fine for hours and hours in Starlight Shores, so I began to wonder what my own world had soundwise that Starlight shores didn't.
I opened my custom world in CaW and removed all the seagulls, flocks of birds, Ocean surf and water buoy spawners, in fact all of the the things that create any sort of background sound effect. Re-exported the world and now all my sims are happily singing their little hearts out, even the karoke machine is now working!
Now, I've no idea why this worked, perhaps it's just coincidence. Maybe the world was just trying to create too many sounds? I don't know. Have tested this theory for a couple of hours and all is still working well, but will need many more hours of play testing to be sure that this was the cause of my silent singers.
