Last weekend I shot some video of an event. Knowing the weakness of the onboard mic, I purchased an external mic with an XLR to quarter inch cable, a quarter inch to mini adaptor, and a mini to stereo mini adaptor. I tested it out before I left home, and it recorded clear audio. You can probably guess what happened on site: one of the adaptors wasn't fully seated, and when I got home and listened to the video file, I realized that there was no audio.
This could have been prevented in many ways, but the most reliable fix would be to monitor the audio live during the shooting. Is there any way to do this with the 7D? I'll get the right connectors in the future to reduce the likelihood of recurrence, but I also want positive confirmation that I do have a good sound signal.