Solving recording problems
Most microphones are mono microphones, so they by default record on the left channel only. To hear your audio on both channels, make sure you record in mono.
If you connect an external audio device (such as a USB microphone or USB headset) to your computer while Audacity is already running, it won't be detected automatically by Audacity.
To solve this, make sure the device is detected by the operating system, then select Transport > Rescan Audio Devices from the Audacity Menu Bar. The new device should appear as an option on the Recording or Playback device selection dropdown in the Device toolbar.
Enable the Software Playthrough option to listen what is being recorded. You will hear the audio through the selected playback device.
Note: you will hear what you are recording a noticeable time afterwards. This delay is called latency. You can somewhat reduce this latency as described in Latency Compensation but it cannot be fully removed if you're using Software Playthrough - you will need an audio interface with realtime monitoring capabilities to completely remove latency when monitoring a recording.
Software playthrough usually causes a delay (latency) of the live recording input due to the time it takes to reach your headphones. To prevent this, turn software playthrough off. This can be done by choosing Transport > Transport Options > Software Playthrough (on/off) and clicking it to remove the check (tick) mark.
To listen to the live recording input without playthrough latency requires hardware monitoring - that is; the input signal must be routed directly through the audio device from the input to the output rather than being passed through software from input to output.
Enabling hardware monitoring is not possible with all audio devices. In particular it can only be achieved when the same device is used for input and output. For example, monitoring a USB microphone through headphones that are plugged into a different audio device will always have latency. If hardware playthrough is not available with your hardware it may be necessary to listen to the audio source directly (acoustically) rather than through the computer.
If you hear crackles, pops, or distortion when the recording is loud, or if the waveform is clearly touching the top and bottom edges of the track, you probably have clipping, which means that the signal has exceeded the maximum allowed level.
Try lowering the recording level using the Audacity Mixer Toolbar or the slider in the operating system. You can also check to see if you can lower the volume on the input source itself (such as the tape deck, record player or microphone). Many sound cards and USB turntables or USB tape decks have an independent volume control for the playback signal level. See Recording with USB turntables or USB cassette decks for more help with USB turntables or tape decks.
When recording, try to aim for a maximum peak of around –6 dB in the Recording Meter so as to prevent the meter's red clipping warning coming on. If the meters are set to linear, the equivalent level to aim for is 0.5. Clicking and dragging on the meter's right edge lets you expand the meter to gauge levels more easily. After recording, you can boost the level safely using the Amplify or Normalize effects.
Help with repairing clipped recordings: If there is only a small amount of clipping (just the tops of a few isolated peaks), Effect > Noise Removal and Repair > Clip Fix can be applied to just the clipped sections. This will attempt to reconstruct the missing peaks by interpolating the lost signal. In other cases where there is mild distortion throughout a recording, using Effect > EQ and Filters > Filter Curve EQ or Effect > EQ and Filters > Graphic EQ to reduce the higher frequencies can help to mitigate the damage. Sometimes a bass cut will help also by making the result sound less "muddy".
A waveform with clipping
The recording slider in Mixer Toolbar is purposely disabled if it cannot directly manipulate the operating system's slider for the sound device, or if that device has no system slider. Turning down the Audacity slider to prevent distortion would be inadequate unless it also turned down the system slider. It would only make existing distortion quieter instead of removing it.
External recording devices such as USB turntables or tapedecks or interfaces may not have an operating system slider, especially on Mac. For all cases where there is no system slider, try to adjust the playback level on the recording device itself.