A lot of audio drivers do mute the signal.
An obvious first is to consult Tascam.
A bit radical solution.
In the past, running Vista, I de-installed all Realtec drivers.
This forces Win to install the default MS High Def Audio Device.
If I remember correctly, this driver doesn’t mute.
Might apply to Win7 as well.
• System: Make a restore point so you can rollback
• Device manager: deinstall the driver (write down type, etc just in case you need to download them from the manufacturers website)
• Programs - remove the driver otherwise it will be installed again at the next step
• Reboot and Win installs the MS High Def Audio Device
http://www.hydrogenaudio.org/forums/index.php?showtopic=60928