I really think that, since this issue appears to not happen while on AC power in the house, attached directly (I'm assuming) to amplified speakers, that this may be an impedance issue. While installed in my car, the Mac Mini's audio out goes directly to RCA plugged into my Infinity 4 channel amplifier. I'm guessing that maybe home amplified speaker sets have higher resistance than this amp, seeing as car amplifiers are designed to take the full signal with little impedance and amplify it. The home speakers must have some additional impedance while going into the volume control module (ah! that's it!) that cause the mini to pump the audio output through the line out jack and not the internal speaker. At least, I think this is what's going on.