For years, there's been some crazy bug in Mac OS X that can cause the speaker balance to become skewed when you least expect it. The temporary fix is to simply go into System Preferences, Sound, click the Output tab, and drag the slider back to the middle... but that really isn't a permanent solution.
Essentially what's happening is, when you change the volume (e.g. by pressing the volume buttons on the keyboard) while the CPU is under heavy load, the OS gets confused and does the digital equivalent of grabbing the wrong knob. In addition to changing the volume, it also changes the balance. Often the effect is subtle, so you may not notice right away, but when you do notice, it's awfully annoying.
Someone came up with a simple hack to fix it: a daemon called simply “Balanced” that runs in the background, doing nothing but waiting for the sound settings to change, then automatically fixing the balance if necessary. I came across several links to Balanced from various forums and blogs while looking for a solution to this problem, but the original web site (on whamb.com) has disappeared, and the download on archive.org's mirror is corrupt.
It took some digging, but I finally found a working mirror where the installer can be downloaded. Also, armed with a working copy of the .dmg file, I found that the corrupt copy on archive.org is simply missing a single null byte from the end of the file, so if you can add one null byte, it works fine.
Anyway, I just thought I'd share.