iTunes Distortion When Turning Up Volume! WTF?

in Genius Bar edited January 2014
I'm getting some serious sound distortion on my external speakers when I use the F4/F5 keys on my 15Al to turn the volume down/up - especially when either of the keys is held down to change the volume quickly. I have some Harmon Kardon Soundsticks. The Harmon Kardon USB is firmly inserted in the port.

Any ideas what's wrong/how I could stop this?
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