What make and model number is the mixer? Does it have a firmware upgrade for Mavericks? What microphone are you using?
It’s interesting that the problem only started with a full reinstall of Mavericks. Some people report non-audio issues with Mavericks like slowness and program crashes after an upgrade install that are solved by a reinstall of Mavericks.
Can you choose Help > Audio Device Info… then copy and paste the information here.
Then can you quit Audacity, open Finder, Go > Go to Folder and type:
~/Library/Application Support/audacity/
Please attach the audacity.cfg settings file from that “audacity” folder as it is now (please see here for how to attach files: https://forum.audacityteam.org/t/how-to-attach-files-to-forum-posts/24026/1 ). Then open audacity.cfg in TextEdit. Select all the text and delete it. Then type the following at the top of the file:
NewPrefsInitialized=1
This gives you factory fresh settings. Save audacity.cfg and restart Audacity.
Gale