When there are students in the queue, the staff automatically take the next student when they're done answering the questions of the current one.
However, when there is nobody in the queue, staff may be sitting around twiddling their thumbs (or on laptops answering Ed posts or many other tasks). As such, they may not immediately notice when a student requests help.
By playing a sound when the a student is added to an empty queue, this would notify the staff of a new question, without needing to constantly look at the screen.
(A sound could be played for every new question, but that could get annoying.)