[Update: This has been fixed in Wwise 2011.1.1]
There is a known bug in Wwise 2011.1 and earlier that causes the Convolution Reverb to output a high-pitched noise on top of the expected sound on NGP and iOS.
This has been fixed for the next release. If you need this fix earlier than that, please contact support: http://www.audiokinetic.com/en/support/feedback-form.