In a new project, Generate > Tone of 30 seconds, amplitude 0.8. Does that play?
If not, does the green playback cursor move through the 30 seconds and does Meter Toolbar show that playback? If the playback cursor and meter toolbar are normal, suppose you export the tone as WAV and play it in iTunes. Does that play?
Another thing you could try is Transport > Rescan Audio Devices in Audacity.
If that does not help, in Finder, choose Go > Utilities then open “Audio MIDI Setup”. Make sure you are choosing 44100 Hz 2ch output.