I wrote a longer post before I answered my question. This shorter post may help others.
I opened a QuickTime movie (.mov) made in 2007, and the audio track didn't play. I did more investigating, and audio wasn't playing for any of my older movies using any QuickTime-based player. My older movies are on a different internal drive than my OS X installation.
The audio tracks of every tested movie played with VLC 2.0.7.
This looked like a weird QuickTime problem or weird drive problem until I discovered that I had no sound at all except for VLC. For reasons unknown, opening the old movie caused AU Lab to close the configurations file that I use with Soundflower. I realize now that I was fooled because VLC bypasses Soundflower. I reopened the AU Lab configurations file. When I opened the culprit movie, it played without problems, and so did all other movies.
