You could plug your speakers into your monitor if your monitor has audio out.
Alternatively you can take the PS3's RCA (the red, white, and yellow wires) and plug those into your speaker system (you might need a 3.5 mm adapter). Then you have to go into your PS3 settings under audio and set it up to output audio through the RCA cables and video through HDMI (the PS3 has the ability to send audio and video to separate places, which is really handy).
Anyways, if your monitor has audio out (such as a headphone jack) then you can plug your speakers in there and it should be fine. Otherwise, get out your RCA cables and run those to your speakers, then change the settings.
I'm currently outputting from the RCA to some 2.1 PC speakers, and it works just fine. I recommend the RCA method because the cables are better organised (you don't have to have your headphone jack occupied and all of the cables are discretely laid out behind your hardware, so everything is neat and tidy).
Hope this helped.