I have a T3i too.

And I know this does not help but it's fine here. HOWEVER I have a few ideas.

1. Switch the camera to one of the creative modes and reset the camera settings back to stock. (try again)
2. Is the recording area very loud? I have yet to record in say a rock band situation but there was a digital camcorder that could not record in loud places. And for that camera the fix was to muffle the sound with sound absorber over the mic. No. I do not know if this applies to the T3i. I only bring it up because it's something I look for.

Hope this helps,