if you will only ever want to listen to one source (DVD or computer) at a time, it can simply be done using a multi-way switch (to change which input you are listening to. The display could be in the form of two LED level meters such **broken link removed**
on a phono, jacket is ground, pin is signal. Red is right, white is left.
on a 3.5mm stereo socket, tip is left, the bit in the middle is right, the long bit by the plastic housing is ground.
to connect two phonos to a 3.5mm socket, connect the left and right signals on the phono to the left and right on the 3.5mm, and connect the grounds together (in this instance all of these would probably be via a switch)
However, if you want to listen to both your DVD and computer at the same time, then listen to nigel, as you will need some kind of amlyfier/mixer. (also, his point about computer speaker quality is worth thinking carefully about)
errmm, TWO video outputs? (usually phonos come in threes - left audio, right audio and video - white, red and yellow). Are you sure that your player doesn't just have two sets of outputs, each with stereo sound, not surround?
also, maybe you could connect the optical output to an optical input on you computer? only that it would be difficult to connect 4 point surround to standard line in on a computer (line in is usually just stereo), but it can be done (i'll explain more if you need me to)
(in summary) If you do only want to listen to one thing at a time, then make a simple switch as described in the first paragraph - it will be much cheaper than an amplyfier.
Just some ideas.
Tim