It depends on the model of the Xbox 360, some white versions of the console had an added HDMI port fitted to the back of the console as a later edition.
I presume yours is a white console with just the Yellow, red and white cables coming from the back but also having a Red, Green, and Blue on the end of the cable. You can buy a Ypbr converter to Hdmi adapter so you can connect the xbox to a monitor and the signal to it will be Hdmi 720p. as for the audio you have two options. Some Hi-fi systems support and Aux audio input so you would plug the red and white from the xbox lead into them. Its the only way I know but someone else may have a better option. The connections are called phono BTW.