One cause could be a ground loop. I don't understand it, but basically if more than one device is grounded, some current will flow and cause a buzzing, which is amplified by your speakers. If your printer uses a grounded 3-prong plug, that is probably the cause. You could get an adapter that ungrounds it (basically doesn't connect the 3rd prong on the plug), but that could be unsafe, especially if you have a laser printer (they use electric charge to deposit toner). Buzzing at 60 or 120hz usually indicates a ground loop. You can try the ungrounded 2-prong travel adapter for your MBP (which is safe), and that might help.
It is also possible that the printer is just making electrical noise on the USB port (instead of a ground loop), probably power and ground. It shouldn't be, and if it is, it might be defective. You might be able to isolate it. I don't know if a USB hub would provide enough isolation, but you could try it. You will need a powered hub so that the printer gets power from the hub's power supply and the computer is not powering the hub. (I know the printer isn't USB powered and still plugs into the wall, but a hub might be able to isolate the +5v signal). Ground will probably still be connected to the computer, which won't do anything if the problem is a ground loop or noise on the ground line. Another thing you could try is a USB through ethernet extender.
You might be able to find a hub that isolates the power and ground (search for USB isolator), but real isolation will probably be expensive because isolating USB is hard:
http://www.maxim-ic.com/appnotes.cfm/an_pk/3891
here's one that claims to isolate completely
http://www.bb-elec.com/product.asp?sku=uisohub4
If the printer is causing noise through the data pins, you probably don't want it plugged into your computer, though, and you might want to get a new printer.
A solution would be to connect your MBP to the home theater system optically if possible. You can get a cable like this
http://www.monoprice.com/products/p...=10229&cs_id=1022902&p_id=1558&seq=1&format=2
If your system only has coax digital in, you can use the cable above and an adapter like this
http://www.monoprice.com/products/p...=10423&cs_id=1042302&p_id=2948&seq=1&format=2
You might even be able to find an optical to RCA adapter, but that is basically the job your home theater receiver is supposed to do, unless it was made in the days before digital audio.
