Can anyone please tell me how to set up (correctly) the Logitech headset on my Mac OS so I can record and hear my sound through my headphones? I can record, but no sound (in headphones) yet?
Thank you so much… 
It would help us tremendously if you read the pink box just above your opening post…
Please provide at least Audacity version, OSX version and preferably also which model Logitech headset you are using.
Set the headset as default input and output in /Applications/Utilities/Audio MIDI Setup with the same sample rate and number of recording channels that you intend to use in Audacity .
Then restart Audacity and use Device Toolbar in Audacity to choose the recording and playback devices and the number of recording channels.
Set the project rate bottom left to the sample rate you chose in Audio MIDI Setup.
If you need more help, please provide the essential information cyrano requested.
Gale