Symptom
You connect headphones to your computer, but the iSub speaker's volume does not mute as you may expect.
Solution
The iSub is a subwoofer-type speaker designed to be used in conjunction with a pair of speakers. Your computer does not distinguish between analog headphones and analog speakers when you connect either to the computer's analog sound output port. Therefore it is the correct behavior for the iSub not to be muted.
The iSub is muted if you connect USB headphones. Alternatively, disconnect the iSub when you do not wish to hear it.