Some model numbers might help but hopefully we can get you what you want or give you some ideas without that
You appear to be describing something like
This says the monitor has a little amplifier inside the which feeds the speakers which is nice (a lot of modern expensive stuff I have had to pull apart and hook into wires for).
The question is then how does your PS3 have audio going in- I am guessing RCA (
http://www.djtechtools.com/wp-content/uplo...a-cable-300.jpg ) tacked onto component or something if you are not using HDMI.
To this end get yourself something like
http://www.caraudiocentre.co.uk/productima...3-5mm-phono.jpg and stick it into the PC (or anything else that has a 3.5mm port) and then into the monitor. If shuffling them around is not for you or you fancy overdubbing then you have a few options.
You can get a swapping box
http://www.kpower.com.au/www/537/files/3-w...ector-thumb.jpg or you can get a dubbing/mixing box.
The first two are a bit annoying (more boxes and probably another power lead) so I have been known to cheat and wrap several RCA plugs into one output
http://img.photobucket.com/albums/v333/leesturbo/phono.jpg (maplins will rip you off but only to the tune of £3 or so) but be careful with this.
You can also spin it the other way (or more accurately use your PC to mix the sound)- fire the RCA into the line in of your PC (usually a light blue port sitting with the speaker port) with a
http://store.audioholics.com/shopimages/pr...normal/1501.jpg (they come in cable form as well which I would suggest you get instead) and then use the first lead mentioned to go from the sound card to the monitor. This is great as you can then play your own music on top of games. The PC will tend to need to be on (or you get to swap out cables) but it is nice.
You can also repurpose general amps and some DVD players for this but that is a different matter.
Main lesson here- by and large phono/RCA, 3.5mm and everything else aside from HDMI and optical/spdif are one and the same so you can usually wind it all together.
The only problem you might have is some monitors are a bit touchy about where their audio comes from (using VGA- no audio from phono and such like) but if it has external speakers it should be good.