Active speakers like PC ones or are they passive like you attach to a home stereo? Active you just need a mixer (google on passive mixer) - active speakers no easy way, you could use a relay to switch them which is powerd by the PC in the cabinet to switch over when its turned on or something, but that would mean you had 2 amplifiers in there to switch between so an easier option can prob be found if you give some details about what you are working with and trying to achieve.