You try to join an Audio/Video conference that is started by another Microsoft Lync 2010 user. The first attempt is unsuccessful because of insufficient bandwidth. In this situation, when you rejoin the conference, your audio and video is enabled automatically.
To resolve this issue, install the following cumulative update:
2467763 Description of the cumulative update package for Lync 2010: January 2011
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.