Open Audacity. Move your computer to where the microphone can pick up the sound. Hit "Record" and play a 10-second audition. Hit "Play" and listen back. If it is too loud, move away from the computer. If it is too quiet, move toward the computer.
Click "File," "Import" if using a pre-recorded piece of audio. Click on the audio clips you need. Each clip will open in a new channel.
Click "File," "New Audio Track." Hit "Play" and "Record" to record along with the previously recorded audio. Once recorded, hit "Save." Because Audacity uses the computer microphone, the sound from the recorded audio is recorded marginally before the live audio. This creates a latency issue. You can fix this with the "Trim" tool.
Click on the first piece of audio. Click "Tools." Select the "Trim" tool by clicking on the scissors icon. Click to the left of the first sound wave. This separates the audio into two portions. Delete the portion on the left. Repeat this process with the second clip so that both have minimal silence at the beginning.
Align the clips. Click "Zoom" to enlarge the first clip. Click on the second clip. Click "Zoom." Drag the second clip to the left so that the first peak of the sound wave graphic lines up with the first peak of the first audio clip. This corrects the latency issue.
Mix the clips. Adjust the "Gain" controls on the left of both channel strips to balance the volume levels. If you want one audio clip to be louder, set the gain higher.