Check your sound card outputs. You will probably have analogue mini-jacks as well as some form of digital-out, but as there are several flavours - including optical - it is simplest to just consult your documentation.
Then see what inputs your plasma screens accept. I have not come across an audio output splitter, but it may be possible to send the signal to more than one device.
But I would ask if it is necessary to connect the audio output to the screens at all - surely you can use the existing PC sound system, assuming you have got some form of 4.1 / 5.1 speakers?
SD