Why can't I hear audio while recording?

I currently am using Mac OS10.3.9. I tried recording with both the stable and beta versions of audacity. I record with a mic that plugs into a regular Mic Jack in the back on the G4 tower. My problem is that I cannot hear sound when I record with audacity. I mainly record rap music, so this poses a very big problem and I have been unable to record for months now. How may I fix this problem?

Your profile says that you have Windows 10. Do you have a Windows machine as well or is your profile incorrect?

Where are your headphones plugged in?

If they are plugged into the on-board sound card, you can enable “Software Playthrough” (see: Transport Menu: Transport Options - Audacity Manual). The down side is that there will be a delay between the sound coming in and the sound coming back out through the headphones. The delay is unavoidable unless you use an audio device that supports “zero latency monitoring”. Most (but not all) USB audio devices support zero latency monitoring.