Use the speaker icon (lower right screen corner) to drill down into the configuration settings.
Right click the speaker icon and then select Playback devices. Find your active Playback device (speakers) and right-click (or double-click) to access the speaker properties.
There are a number of tabs and settings that you can control. Work through all of the tabs and settings very carefully to see how they are set.
For example under the "Communications" tab you find settings related to muting.
Try changes one at a time and keep track of your change so you can reverse it if necessary.