One thing to check to make sure you don't have a cable plugged into the K3s Line In jack. If you do, that will shut off the signal from the USB audio interface.
Check Your Audio Settings
You should be driving the audio so you have 4 bars illuminated with the 5th bar flickering on the K3S ALC meter. If you are operating in USB mode, I suggest you change to DATA A data submode. That will automatically set compression off and the equalization to flat.
You have 2 controls to adjust the audio drive. One is in the computer, set the slider for the soundcard in use to about 2/3 of full scale for starters, and set the MIC SEL to LINE IN - the MIC Gain control in the K3S will allow you to control the gain of the LINE IN signal.
If you are using the internal soundcard in the K3S, make certain you do *not* have anything plugged into the K3S LINE IN jack. If you do, the LINE IN audio from the internal K3S soundcard will be ignored and whatever you have plugged into the LINE IN jack will take precedence.
You can adjust the audio levels by placing the K3S into TX TEST mode and no RF will be produced. Once the audio levels are correct, you can adjust the desired power level with the POWER knob.
Once the audio levels are set, adjust the VOX gain for reliable transmit.