Every sound card has mixer software of some type.
Regardless of whether it's fancy or plain, its primary job is to let you manage the individual volume of each of the various inputs, adjust the master volume, as well play the treble, bass, and fader controls. As you can see from Figure B , there are also mute options that can turn one or all of the inputs off. That's your next stop. That's usually the indicator that the sound has been muted. You might as well take a look at the volume levels.