When I use the headphone jack I only get sound out of 1 channel. What can I try? I'd appreciate any help diagnosing / fixing this.
I have an HP Pavilion dm3t notebook.
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The first place I would look (after trying a different set of headphones) is the sound preferences setup (System -> Preferences -> Sound). Go to the "Hardware" tab and verify that your sound device has a reasonable "Profile" setting (mine is set to Analog Stereo Output):
Then, on the "Output" tab, verify that the "Balance" slider is centered, rather than all the way left or right: