To resolve this issue, apply the following update:
Description of the Communicator 2007 R2 hotfix rollup package: April 2009
After you install this update package, you no longer experience one-way audio. There are known issues with the LG-Nortel IP8540 and Polycom CX700 devices when they are used in AV calls on some Windows XP-based client computers. These known issues are scheduled to be fixed in a later update of Microsoft Office Communicator 2007 R2.The third-party products that this article discusses are manufactured by companies that are independent of Microsoft. Microsoft makes no warranty, implied or otherwise, about the performance or reliability of these products.