Buzzing on computer speakers when LEDs are on I have an Arduino Nano connected via USB to a Mac Mini. When the LEDs come on, an external speaker connected to the Mini starts to hum/buzz quite loudly. If the LEDs are turned off, the buzz stops. 

The speaker and the Mac mini are both powered independently, and are plugged into one wall socket, while the 5V power supply to the LEDs and the Nano is plugged into a different socket (of the same outlet).

What could be causing the buzz and what are the possible remedies?
If this is the same project you described in this question(readers: "112 WS2811s in series" causing interference), your external speaker may not be filtered very well against energy radiated by nearby un-shielded wiring. Try a pair of headphones in place of the speaker - does it also buzz? If not, the speaker (system) is probably picking up the interference. If so, the pickup is probably within or near the Mac mini.

You could try a few simple solutions (or at least tests that might narrow the problem):


shielded audio cable from the Mac mini to the speaker;
shielding the speaker enclosure and grounding the shield;
shielding the connections to the LED string (obviously shielding the LED string itself is likely to be aesthetically undesirable);
separating the speaker from the LED strip by more distance; 
powering the mini from batteries - at least long enough to learn whether the power wiring might be involved.