[Update]
Each time after using Siri the sounds are at low volume again, BUT after a few seconds (guess when the Siri process is determined automatically by iOS) the tones are back to the adjusted volume set in Tones settings.
When using MAPS and especially turn-by-turn navigation (with voice response) the sound issue is reproducible present. I guess this is caused, because voice response in MAPS is using Siri's Voice (Process) to give the according instructions. If not shutting down MAPS, somehow the Siri process is still working and causing the malfunction.
The only thing you have to do is shutting down MAPS. You don't even have to do anything else! No shutdown, no hard reset, no factory reset, no adjusting of volume...
Just open up the "task Manager" by pushing Homebutton twice. If MAPS is running, shut down the MAPS Process and wait a few seconds (about 10 seconds). Sounds are fine again!
This works fine for iPhone 4S with iOS 6.0.1
Anyone can confirm?