panta rhei wrote:Now, what SEEMS to happen is that Audacity interferes with System settings and disables the mic.
As you will have gathered, I'm not a Mac person (Windows at work and Linux at home), but I sensed your panic and frustration and I know how frustrating it is when a computer suddenly goes wrong, so I thought I'd jump in and try and help while you were waiting for the Mac elves to arrive.
As far as I'm aware, Audacity does not actually make any changes to the computer sound system, other than perhaps adjusting the record/playback levels (this is certainly true on Windows and Linux). However, the simple act of plugging in a USB audio device
does cause changes to occur. On Windows it is particularly annoying as by default Windows will often automatically hand over the entire sound system to the USB device (recording and playback) and set the recording input to mono. What that means is that all of a sudden, the computer looses all sound (because a USB turntable is a "recording" device only and is incapable of working as a "playback" device). This often happens when someone has just bought a USB turntable and plugged it in for the first time, and they don't realise there is a problem until after they have tried to use Audacity and cannot get any playback. Consequently, Audacity often gets the blame, even though the actual "fault" was due to plugging in the USB device and how Windows responded to that.
On Windows, the solution is to go into the Windows Control Panel and set the sound system to use the normal sound card as the playback device, set the USB device so that it is enabled as a recording device, and set the normal sound card as the default recording device. This configuration will be remembered after a reboot. Other applications will continue to work normally, and Audacity can be set manually to record from the USB device via the settings in "Edit menu > Preferences > Devices".
There are obviously some differences on Macs (which I can't help you with), but from what Koz was saying it sounds like
"Apple (upper left) > System Preferences > Hardware > Sound > Input and Output" is doing a similar thing to what a Windows user would do in the Windows Control Panel.