One of the features of SmartSDR for Windows is the ability to use a microphone and headphones connected to the PC with your FLEX-6000 which is described in the SmartSDR Software User Guide.
Here are some troubleshooting steps if it is not working correctly.
1.) The headset and mic must be the Windows default sound playback (headphones/speakers) and recording (microphone) device.
The HelpDesk article, How To Set The Windows Default Playback & Recording Audio Device describes how to set the headphones and mic connected to the PC as the default Windows sound devices.
2.) You have to enable the PC Audio feature by clicking the PC AUDIO button on the top right-hand corner of the SmartSDR for Windows console.
3.) You have to select the PC microphone as the audio input for the radio by selecting "PC" in the Mic Selection dropdown box (see section 22.2 for the details on this dependency.
4.) The last thing that you need to check since you are using Win10 is to make sure your operating system has the microphone enabled. Please see the HelpDesk article Enabling the Win10 Audio Recording (Microphone) Devices for instructions on how to do this.
5.) If the DAX button in the P/CW window is illuminated, click it so that it is off.