I'm assuming you mean you have HDMI coming out of the PS3 and going in to the DVI port on the back of the TV, via an adaptor or cable with a different connector on either end.
I have the same TV, you need a separate audio connection between ps3 and tv.
HDMI can transmit audio and video but DVI (which is what the TV has) can only do video.
You'll need a 2 x phono female to 3.5mm jack male adaptor cable.
like this one
to make the audio connection.
you could also try this adaptor
but its a bit big and therefore may not fit into the space available.
after getting an adaptor do this (keep your original setup the same)
1. Put the ps3 AV cable into the back of the ps3.
2. then connect the red and white plugs (ignore the yellow) from the ps3 AV into the adaptor (discussed above).
3. then connect the 3.5mm jack (from the adaptor) into the small audio port right next to the DVI port on the back of the TV (I believe its called 'DVI Audio')
I've not tried this and I don't have a ps3, yet, but in theory it should work, as long as the ps3 will send
audio to the av output at the same time as to the hdmi output.
if it works, then post a response, i'd like to know for the future.