If you install itunes, OBS will automatically detect and use the CoreAAC encoder, which gives noticeably better audio quality for the same 160kbps bitrate.Ripping out all the audio drivers tonight and reinstalling. Got the video right and all the OBS stuff as I need it. Nearly there!
If you don't wanna install that junk software (like me) shoot me an email - firstname.lastname@example.org - and I'll send you just the encoder installer which I ripped out of the itunes installer.
If you check OBS log files in the Help menu, search for "CoreAudio". If you find it being added at startup, it's already installed and in use.