[ubuntu] Acer 5742-6874 Only One Speaker Working (Oneiric)

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http://ubuntuforums.org – Hello everyone. I've recently installed Oneiric on an Acer 5742-6874, and only the left speaker is working—the right on-board speaker is not producing sound. Both left and right speakers work when I have it set to HDMI out and run it through my TV; ditto my computer monitor. Also, both left and right headphones work when I have a set plugged in. Finally, just before installing Ubuntu, the on-board speakers worked in Windows 7. How do I go about diagnosing why the right speaker isn't working? More importantly, how do I fix it? Cheers! Specs for machine: Processor (Hardware)