I have a problem with sound playback on 4 month old mac pro. I have not upgraded the sound card, it is the mac pro standard one. I am suddenly experiencing no playback at all from itunes or safari or anything. I tested my external speakers and it is not them, they work fine. I have done system preferences > sound > made sure line out is selected. When the mac starts up i still get a 'dong' sound that comes out the internal speakers, and also i do get playback from the internal speakers when internal speakers is selected in sound preferences. My question really is how do i know if my sound card has blown, and if the internal speakers ARE working, does this mean the sound card has not blown?