Here are my results from a series of tests run on Studio Mode audio in Google+ Hangouts On Air. The results come from the archived video on Youtube, not from within the Hangout itself.
Update: August 2014
What is the sound like in HD Hangouts?
Google upgraded Hangouts On Air to HD video a while back, and with that upgrade have come some improvements to the audio. This test is a bit more in depth than the original from April 2013, including audio files for you to hear the difference.
Testing
Playback of a high quality audio file of one of my original songs (a demo from 2012), first at average around -10db, then at full volume with average level around -2db. Provided is the original 320kbps MP3 file as a reference. At -10db you’ll be able to compare and contrast early Studio Mode (archived test from August 2012) with current Studio Mode. Also provided is a a test of Voice mode with the high quality filed played at -10db.
As another point of comparison, I’ve included audio from the same song clip played through Youtube Live.
Then I ran a series of frequecy sweeps through Hangouts On Air, going from 20hz to 20khz, at various levels of loudness: -3db, -5db, and -8db.
Spek is used to analyse the audio files and generate spectrograph images.