Media center supports up to 5 extenders, so its possible to play to 6 locations (including the media center itself at the same time).
You can achieve this for audio, but even a 100mb lan network isnt cabable of streaming 5 HD videos.
Its getting easier with DNLA devices, such as the Xbox 360 and the Playstation 3 to connect to other audio hardware that also support the standard.
The biggest problem you will face is trying to get the same sound out of each of the extenders. All extenders buffer the data in an attempt to cope with network traffic, so theres no guarantee that standing between rooms where theres an extender in each room wont sound really strange.
The only way to achieve this with a perfectly syncronized audio track is to use some specialist hardware, such as the Airport express wireless speakers or Sonos audio system (Nokia have a similar product called Nokia Home Music). They are not cheap though.
Edit: I came across a blog entry today that describes using normal telephone wires and sockets to move music around the house. Might not be directly relavant but I thought I would share with you just in case;-)
Link Here
Ian Dixon at the digial Lifestyle has a podcast this week about sending audio via DLNA and networking some tuners. This may help....
Podcast link