Users have reported that sometimes the sound works fine, and other times the sound is muffled or nonexistent. Some users say that a temporary fix can be found by covering the noise canceling microphone. Others have suggested using a third-party video recording app.
And while HTC hasn't specifically identified the source of the problem, we can infer that it has something to do with the noise canceling, in that the ThunderBolt is canceling the desired audio source as if it were ambient noise.
HTC is confident that a fix will be out soon, but we won't be surprised if it's merely packaged with the ThunderBolt's Android 2.3 update, along with the Bluetooth fix, coming within the next few months.
source: Android Central