first, please instead of digging up an old dead post that's slightly related, start a new one
You need a router, not a switch (the extreme is one example of a router)
if you only have a switch, than everyone has access to anything that is wired to it. (with the right address, i could access your TV with no passwords) Plus you have multiple devices connected to you modem, most ISPs limit the number of connections you can have. If you have a router, it will share one connection to the outside world to all of your devices.
and multiple more security issues from using a switch instead of a router.
you can have an extreme (or any other router) and an express running on the same network, so you wouldn't loose your music functions.