I've had it happen with a bunch of cars, on random tracks, both on and offline. I've had no engine, just transmission/turbo noise and tire squeal for laps upon laps. If I randomly hit something or run over curbing, my engine is back. Sometimes, I don't think I did anything, just shifted to the next gear and it had sound.
I've had sound cut in/out mid race temporarily, for a few seconds at a time, no rhyme or reason.
I've also had all sound, including effects and music, even the menu "clicks", disappear. Just cut off entirely mid-race. Persisted across cars, rooms, offline mode, practice, menus, etc. Had to quit and re-load to get back sound, sound came back as soon as the XMB loaded.
I can't tell what's causing it, how to prevent it, or a surefire way of resolving it. I have a nicer Yamaha 5.1 receiver that's provided years of trouble free service, I'm 100% sure it's not the hardware.