For a while, I had noticed that suddenly, the system sounds of my Mac would not play. For example, whenever I am chatting with someone on iChat, for every message that is typed by me or the other person, the Mac usually plays a short sound.
At one point, these and other sounds would totally shut off. A restart of the laptop would resolve the problem.
Recently, I figured out that this happened as soon as I muted the sound on my Mac and then put it back on. After this, the system sounds would totally shut off.
So, now I do not mute the sound totally. I just reduce the volume in case I need to do that.
The system sounds are really important because they alert me for different events. For example, if I'm chatting with someone and I start work on an email or a document and the other person types a message I would not know unless the sound plays because the chat window is obscured by the email or document window. The other person would think I am ignoring him!