According to Apple's description, the aurioTouch sample app is suppose to "get the audio input and copy it to the output", which I think means that the app will playback/thru any sound the iPhone's mike picks up through the iPhone's speakers.
When I loaded the app onto my iPhone (updated to 2.2), however, the playthru feature does not seem to work at all. The on-screen oscilliscope reponds as expected to voices and sounds, so the audio input half of the app is clearly working.
Yes, I have tried toggling the app's "MUTE" button, and yes, I made sure the iPhone's "ringer/silent" switch is not set to silent, and that the volume is not set to zero. What am I missing?
The issue is an incorrectly written DC filter in the helper code.
Comment out lines 210 and 211 of the AppDelegate code, which invoke the DC filter, and you'll be up and running immediately.
来源：https://stackoverflow.com/questions/312089/auriotouch-sample-apps-audio-playback-thru-not-working