This is not in the design. There are a couple of workarounds depending on the kind of device you have.
If your device requires a cable connecting from the TV card to the sound card (you would have checked the "This device requires a cable to route sound directly to the sound card" checkbox), you can just open the Windows Master Volume control and mute the line to which the cable is connected.
The other workaround requires you to record from Media Scheduler, instead of manually start recording. For scheduled recording, you can edit the value of "Want Preview" at HKEY_CURRENT_USER\Software\JRiver\Media Center 11\TV Settings\. Changing the value to 0 will prevent TV (both video and audio) from showing during scheduled recording.
EDIT: The second option above is available on Options\Recording for MC12. For MC11 you have to use the registry workaround.