In order to mark Audio and Video packets for DSCP in OC 2007 R2 – the following steps has to be performed on Vista SP1 and SP2 PCs: 1. Create and update the following key: (32 bit DWORD set to) a. HKEY_LOCAL_MACHINE\Software\Microsoft\RTC\Transport\QoSEnabled ; b. Set 32 bit Dword to 1 to enable ; c. Reboot PC – (this will not take effect until rebooted). After the reboot, RTP Media will be marked with the following default values: (which can be changed) 1) Audio calls will be marked with DSCP 40  2)  Video will be marked with DSCP 24

Full Article: OCS blog