I've had the same problem, and the only answer I've found is to turn the sound down manually.
Some games are more well behaved about it, and recognize that ringer off means sound off, some of them don't care and just charge ahead unless you turn the system sound down while in the game.
You could try the silent night adapter. I'm not sure if it will work but it might....it mutes everything else except calls and alarms. I have one I use in my dock next to my bed at night but it can be plugged directly into the headphone jack. It's super tiny so it won't get in the way.