Starting with SmartSDR v1.4, the LAN Remote feature uses your PC's default sound playback and record devices as the remote speakers and microphone. If you are using a USB headset, you will want to 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.
If possible, 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.
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 or your PC