Hello Liz,
Just to clarify, you state QuickTime is not playing audio on any of your files then mention they are clips from your AVCHD camera. Does QuickTime play other video files not from the camera (or even audio files, like MP3's)?
There are sample files available for testing here:
QuickTime Sample files - Apple Support
Are you using QuickTime Player 10 or 7? (Not sure what updates you did with your MacBook Air.)
If QuickTime Player can't open an audio or video file - Apple Support
QuickTime Player (version 10) can create, play, edit, and share many media formats, including many that are no longer in common use. In OS X Mavericks and later, QuickTime Player converts certain older formats that it doesn't play back directly.
If QuickTime Player doesn't open your file, search the Mac App Store for a third-party media player that supports the format of your file. You can search using the format name or the filename extension (such as .avi, .wmv or .mkv).
If you are using QuickTime 10, does QuickTime 7 still play them okay?
Installing QuickTime Player 7 on your Mac - Apple Support
QuickTime Player 7 may help if you have older media formats or workflows that require an older version of QuickTime Player.
If QuickTime Player (version 10) doesn't work with your older media formats or workflows, the legacy QuickTime Player 7 may help.
- Make sure that you're using Mac OS X Snow Leopard v10.6.3 or later.
- Download QuickTime Player 7, then double-click the downloaded file to open it.
- Double-click QuickTimePlayer7.6.6_SnowLeopard.pkg and follow the onscreen instructions. The installer puts QuickTime Player 7 in the Utilities folder of your Applications folder.
- If you have a QuickTime 7 Pro registration key, you can now enter it now.
If none of that helps, can you give us more details on the versions you are using and if you are getting no sound with all files or just the ones from the camera?
Thanks.