LOL...
I think it's funny that you can ask such a simple question and nobody will give you an answer...
Anyway, I have the same issue that you are having...no sound on my 360 (even though you are using an original xbox) when I use it w/ my 2407WFP. What you will need to get is a composite sound cable extender. It has the red and white females on one end and the red and white males on the other. This, of course, is provided you have a receiver on the other end that will accept red/white audio. So if your audigy has red/white (composite sound) in then your sound will come over your PC speakers. On the other hand, if your sound card has only line in, then your cable will be what the previous poster showed you. However, it is most likely that there will not be one long enough to suit your needs so as you have already stated, you will need a 2nd extention but the second cable will be a line-in extender cable. This goes into the line-in jack on your sound card and in your audigy driver you will have to disable mic and enable line-in while you are using your XBox. Does that make sense?
The other alternative you have is that you could just go to Dell and buy the speaker attatchment that they sell with your monitor. It's pricey but it could be worth it cause it fits on the monitor like it's supposed to and there is a built in jack on the screen for this (I think, I have both the 2007WFP andthe 2407WFP and I know that atleast one of them have speaker upgrades but I thought both did). Also, you could look for an additional set of 3rd party speakers that accept Red/White composite or a set that accepts line-in and then also get the coverter cable the earlier poster showed you.
Good Luck,