OK, start some music playing through the internal speakers, then open system preferences / sound / output. Now, move the balance slider from 100% left to 100% right. Do you hear a difference in the sonic character? It may be subtle. On my MPB, my left speaker is quite a lot brighter (i.e. more treble) than the right. A design quirk to minimise feedback perhaps? Ok, but why is my 2yo PB brighter on the right then? It annoys me because I like to use the internal speakers to reference a mix but it's totally misleading. Thoughts?