If the monitor does not have speakers then it really does not matter what cable he buys,the monitor will still not have speakers.
That monitor is bound to have a 3.5mm port for headphones. If your speakers for some reason don't have a 3.5mm jack attached, go to an electronics store for another cable.Just connect the speakers to the subwoofer, and connect the sub to the monitor. At this time the XBOX 360 doesn't support connecting with audio ports, so it has to be done via the TV/monitor. For your information, there is no audio port for speaker connections on the 360. The XBOX can't be connected to speakers, because all the XBOX does is produce the picture and sound for the game and send it to an external source, i.e. your monitor, so anything to alter picture or sound has to be dealt with on the monitor/TV. Good Luck!!