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You can import the video to iMovie application and then add or record the audio using iMovie and place it on the video. Once you did that, you can export or share the project to final video.
Open iMovie from Applications folder. Click on File > Import. Import the video you recorded using Photobooth. Then record the audio and export it.
Refer this article --> https://support.apple.com/kb/PH14708?locale=en_US
See this tutorial video --> https://www.youtube.com/watch?v=4o6KvlteJho
If you have any difficulty on following the steps above, please let me know and I will continue to help you
If you didn't turn on Mic, then that means the audio isn't saved at all. It is just the video that is recorded and you will have to re-record the audio.
Can you try playing the video with this VLC Player and see if it has the audio ? http://www.videolan.org/vlc/download-macosx.html
I'm sorry about that. It is supposed to record the audio along with the audio by default.
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You should also set correct input device on Apple > System Preferences > Sound. So it won't happen again.
Also run Disk Utility and repair the disk with it. Let me know if you need with help that.