I just got the canon t3i in the mail today, my first DSLR, and I'm trying to record audio. Using both an external stereo mic and the internal mic, there is a constant loud hiss. There is even a hiss when I plug in the external mic and don't turn it on. I don't think it's the AGC, because the hiss is still there when I change the settings to manual.Thoughts?

That's alarming. I'll let you know when mine arrives if it has the same problem.That said, I tend to think of DSLR video as primarily video acquisition; a dedicated camcorder is likely to capture a better sound/picture comp.

I have no hiss on my T2i and am guessing that the audio setup is same for both models. Treat it as a defect and get it back to dealer or Canon. There should be no hiss unless you are taking video of, perhaps, snakes. -- joel albert

Some external mics have a "high pass" filter -- the purpose of which is to eliminate the hiss. The Rode video mic has one (http://www.amazon.com/Rode-VideoMic-Directional-Condenser-Microphone/dp/B0007U9SOC/ref=sr_1_1?ie=UTF8&qid=1300679974&sr=8-1)Do you have access to any mic equipped with a high-pass filter that you can use to test?

Here is a sample of the audio hiss. Can anyone help?http://www.youtube.com/watch?v=T5pL0gJkeKE

Doesn't sound like the AGC as I cannot hear any "ducking" or gain pumping when you speak . IMHO take it back to the shop .