Starting with SmartSDR v1.4, the LAN Remote (now called PC Audio) feature uses your PC's default sound playback and recording devices as the remote speakers and microphone. If you are using a USB headset, connect it to your PC and configure it as the default sound device.
Also, if you are using VAC or DAX to operate digital modes with your FLEX Software defined radio, you may need to change the default sound device that Windows automatically assigns when new sound devices are added to your computer.
You do not want a DAX or VAC sound device to be the default Windows sound device because that could result in QRM if Windows plays sounds through your digital mode program while you are transmitting. If using the SmartSDR PC Audio feature, DAX cannot be the default Playback audio device. If you see a small exclamation point (!) in the PC Audio button, that indicates that a DAX device is one of the default Windows playback devices, and you need to change it to get PC Audio to work correctly.
Both of these are valid reasons to change your PC's default sound device.
The link below is a YouTube video that describes how to change the default sound and default communication sound device on your PC
For Win10:
How to change the default playback and recording audio device in Windows
For older Windows operating systems