I'll chime in. I bought three of these Acer Aspire One's for my kids for Christmas. All three were the Windows XP OS, with the 120GB hard drive. On all three I installed the same after-purchase software: Mozilla Firefox, Webroot anti-virus, Malwarebytes and AOL Instant Messenger. The Acers are not used for anythig heavy, pretty much just the AIM and YouTube videos.
I had this same sound problem develop on one of them within a week. I did a system restore and that fixed it, but I put the same software back on. The problem returned so brought it back to the store and exchanged it. The new has been fine, and I put the software back on it.
Acer #2 developed the sound problem last week. I tried to find a sound card driver update, that didn't work. I reinstalled the factory sound card drivers, that didn't work. I just did a system restore tonight and that fixed it. Now I'll slowly ease in the software to narrow it down.
The one thing I get suspicious of is Firefox. It asks for a Flashplayer update, and I wonder if that has anything to do with it? We'll see.