Hi John,
There's a setting in the Windows sound options that allows applications to take exclusive control of the audio device. I recommend having this turned off it isn't already:
1. Go to the Start menu and open Settings->System->Sound.
2. Click on "More sound settings" and then under both the Playback and Recording tabs, click on each device being used with FreeDV and then click Properties.
3. Go to the Advanced tab and uncheck "Allow applications to take exclusive control of this device".
4. Click OK and repeat 2-4 until you've confirmed this setting on all devices used with FreeDV.
5. Click OK on the main Sound dialog to save the modified settings.
Let me know if this helps for you at all and if not, we can continue with further troubleshooting.
Thanks,
-Mooneer K6AQ