I use almost 10 tabs at the same time, because I want to keep important websites in my sight, and sometimes YouTube on several tabs: one for tutorial, one for music, and so on. In this situation, the speaker icon indicating which tab is playing sound is quite useful.
Let me give you another example. Some websites play sounds (with music, video, commercial flash, etc) without notice and make you embarrassed. I have an experience that one of them played background music without ANY visual indicators: no play or stop button. This situation is very uncommon, but this small icon helps you out from this uncommon, embarrassing experience!