Unable To Mute Computer Built-in Speakers

I am running the latest version of Windows 10 on an HP all-in-one desktop computer (27-1015t);

When listening to music, from the computer's CD drive or from iTunes, I like to turn off the computer's built-in low-fi speakers and instead listen to a pair of external powered speakers, which are connected to the computer's audio out connector;

Before going from Windows 7 to Windows 10, I could mute the built-in speakers without muting the output from the audio out connector;

Now under Windows 10 I can't find any way to mute the built-in speakers without also muting the output to the external speakers. One might think that the connector where I plug in the external speakers jack would automatically disconnect the internal speakers, ( as is true on iPhones, iPads, and the like) but that doesn't seem to be the case.

Is there any way under Windows 10 to play the external speakers only? Or is that HP's problem rather than Microsoft's? Since I *was* able to do it under Windows 7 but I can't now, it seems like a problem relating to the OS change from 7 to 10.