I have just bought this computer about 1 or 2 months ago. It has been fine the first few times I used it. The computer had sound normally e.g. when I played games or when a message popped up at the bottom right hand side of the screen. However for the past few times I used the computer, I realised that it does not produce any sound anymore. Even when I played music, there were no sound although the Windows Media Player showed that the music was being played. I looked into the troubleshooting manual and checked the whether the sound card was faulty but it showed to be working properly. In addition, I checked the connections to the CPU of the speakers and even tried plugging off the speakers but the result was still the same. All in all, the computer has no sound.
If windows XP (i think for vista is pretty similar)check in "control panel" > "sounds and audio devices" - just to check the sound is not muted or turned right down. Check the speakers on another PC, and check your PC with other speakers / headphones to see if it sheds any light on the problem. Update or reinstall the most updated drivers for the soundcard, this may solve the problem, you can get them from this link; http://www.acerpanam.com/synapse/forms/portal20.cfm?website=AcerPanAm.com/au&siteid=8116&areaid=12&formid=3394#results Hope this helps :)
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