Hi there Stella,
Your requirements and thoughts are along the right tracks along with Dar's input as well but theres a number of extra elements to get all of what you want/said
1. The basic 'Wireless' network functionality can and is provided by the 'Airport' and many other manufacturers technology as well - it's name is 'Wi-Fi' (Airport is just Apples marketing name for it) and depending on other devices and their application, you can 'do' allsorts with it!
2. To get music on external speakers, you need those speakers to be connected to a 'unit or device' that generates the sound. In turn, it needs a 'source' of music. Tradiionaly thats been a CD player, Cassette Player, Record player etc and Nowadays an MP3 player. Most Mp3 players are not 'Wi-fi' equipped, but you can get something like the "Roku Soundbridge"
http://www.rokulabs.com/products_soundbridge.php and/or buy a wifi equipped 'base station' that you plug your MP3 player into so it 'sends out' its contents via the Wireless network.
So typically, you have lets say 4000 songs on your main PC, connect the Roku to it, and then that will wirelessly 'pipe' it to recieving devices around your home - they in turn, connect to your audio system which of course is connected to the speakers and - voila, you have music (or whatever you listen to!) - there are other setups and configurations but that is mainly the gist of it
3. Do the same above with Video files as well and think of it as a wireless cinema !
4. Be aware you cannot use this setup or Wifi to print wirelessly. In order to achieve this, you need a) a printer with a Wi-Fi reciever built in or b) whats called a Wireless Print Server.
As everything, all the manufacturers make devices which can do all of the above and more. There are performance variables as well, other elements such as the thickness of your walls, distance, possible blocking objects can all reduce the effectiveness of the wireless range and subsequently performance. Wi-fi is great when it works and a pain in the butt when it's 'trying to work' and you get 'jitters' in the signal etc.
5. One very important aspect is wireless security - I wouldnt recommend leaving the wireless network 'open' so mates can come in and surf on your connection - people still do drive around picking up open networks to see if they can get access to important stuff on your PC's after all if the 'front door is open, chances are you financial and ID is lying around with easy access' - you MUST lock down your network.
Sounds like alot to do, but once you do some research and reading you'll find it 'relatively' straight forward to know what you want, the particular products to do what you want and then get it setup correctly - problems are mainly in the setup and security and performance aspects really.
Once working you'll like it!
As an example I've had Wi-Fi now since about early 2002 and its expanded so that it's my internet connection amongst about 4-5 laptops and a couple of desktop machines, A Wi-fi capable printer shared amongst all machines, a 330 GB library of MP3's for distributing Music through those PC's (soon I want to get a Roku type unit) and a store of about 300 Hollywood films in mildly compressed format so they stream over the wirless network better. It's fully locked down so access is only given by me explicity to 'guest machines' - also, it's ready for when/if I do buy newer devices - in addition to the Roku type unit, I want a Wireless media centre so I can stream all my content direct to my living room TV as well.
Hope some of that helps!
Cheers! DtM! West London & Slough UK!