What is happening/going on when that symbol appears? It looks vaguely like the mute symbol - are you perhaps accidentally hitting the mute button when using the keyboard? What OS are you using?
Check the Sound preference pane and look at the output section. has something shifted your sound output over to digital. I normally have my Mac Pro set to output to the speaker so it can be routed to the stereo. Every once in a while though the selection will be shifted to the digital output and because there is nothing connected to the digital output there is nothing to change hence the mute.