Actually, the situation is worse than I thought. Today I drove for about 6 hours, mostly listening to music via the headphones port (plugged into my aux input on my car stereo). For the life of me, I could not figure out how to get ringtones and alerts volume to be loud enough. Trouble is, there are really six volume settings:
Phone speaker media and ringtones/alerts
Headphones media and ringtones/alerts
Headset (ie with microphone) media and ringtones/alerts
And, of course, you can only access two of them relatively easily. I think today, somehow, my ringtones/alerts volume took on the media setting from my headset, which is pretty low. That's the only one I couldn't change while driving. I think the ringtones get to pick one of the six at random.
I did the reset all settings maneuver, and that seemed to do the trick, but we'll see how long that lasts. It was also a lot less painful than I thought it would be. All my email accounts and passwords were restored, for example.
Apple, please kill this bug. You guys have done audio so well, this minor bug is really quite beneath you.