Has anyone tried a master reset to see if that clears up the issue? I say that because alot of these types of odd issues seem to clear up post OTA's via a master reset (some carry over from the upgrade?). I'd try it, but I'm not experiencing the issue (I do have the GB OTA).
I'm suspecting it's an app conflict that might be changing the volume inexplicably. Perhaps a media app, or some other app. I dunno... No doubt, since a reset would remove all apps (initially), that might "cure" it. For a while... Like Mortiel, I've noticed this phenomenon since I first got the TBolt. But it is way more frequent since updating to GB. But, I have also installed more apps since then. And... it looks like a lot of folks have this issue. I'd guess it's systemic to the OS. Both Froyo and GB but worse with GB. For me anyway.
Personally, I don't have the "volume too low" issue. Although, I have noticed certain volume settings are a little lower after the update. But there is still plenty of higher volume adjustment left for me to make it more than loud enough.
The only issue I have is that when I'm least expecting it (more often than once a day) my volume will blast me out. When I'm checking voicemail (visual voicemail) and when I answer the phone. It's so loud any other person near me can hear it and it hurts my ear. I have to pull the phone away from my ear it is so loud until I lower the volume. When I check the volume it is at max (where I never set it, ever -- just for this reason).
-Frank