I'd say it's probably the speakers. Perhaps they are slowly dying? Do you have some other speakers/headphones to try when your computer just plays though the right channel.
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Primary- i7 920 | 6GB RAM | 9800GT | 160+160+500GB
Secondary- Athlon X2 4200 | 2GB RAM | 7600GT | 80+250GB
Netbook - Atom N280 | 2GB RAM | GMA945| 160GB
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