I just did this last week. My hi-fi is also connected to my tv and sound comes through everywhere as long as that particular channel is on.
Basically make sure you have an s-video
http://coloredhome.com/video/s-video_luma_croma.JPG connection of both your tv and laptop (laptop does have it but tv doesn't say so check). If you do then you can just get an s-video cable.
For sound you need a3.5mm Jack to 2 x RCA phono stereo as pictured
http://www.martleyelectronics.co.uk/...rca-socket.jpg . Make sure you tv has that red and white connector as shown here.
Let me recommend you to get them of ebay as these wires are very cheap compared to getting them from a shop.