I have bought a number of the Plantronics PC Headset speaker switch that JamesL mentioned for my family and friends and they work great. I would just like to mention that the wire that they have are thin and not shielded, so they are prone to interference if you jam the wires with the monitor dvi cable, power, and etc.
I recently bought a new computer monitor that has internal speakers so I got another Plantronics switch to change between the monitor speakers and my ATH-A700 for convenience. When using the A700's I do notice a lot of hissing. I have not tried the switch for a few days nor have I tried to move the switch cable away from the rest of the bunch.
PS, I got them in the past at RadioShack. The current one was purchased at Office Depot for $13.99