I'm thinking about using an Arduino as an audio switcher, like one audio source (a PC audio card) and multiple small speakers, like one per room, so you can have cheap music diffusion in your house.
For the sake of simplicity I don't care about adjusting the volume of each speaker, but just turn it off or on.
Any ideas on how to do this?
I found a nice page with several amplifiers...
http://www.techlib.com/electronics/audioamps.html
Is it suitable to have one amplifier, and so switching a power signal to the speakers, or switching the audio signal and handling it with an amplifier for every speaker?
If you're switching speaker-level audion signals, just about the only way to do it is with relays.
What kind of relays do you suggest? Probably the solenoid ones are the cheapest but maybe they introduces noises...
I don't know what you mean by "solenoid ones", but the main consideration is the current rating of the contacts. How many Watts is your amplifier, and how many Ohms are your speakers?
Sorry I mean electromagnetic relays 
The speaker could be something that you find in cheap computer speakers... like this one, I think they usually are 2W 8 Ohm parts.
The amplifier could be a 20-30W one, so it can handle easily 8-10 mono speakers.
It would be great to have a half duplex communication system, with a speaker and a mic in every room with just 4 wires (power and signal)... I think it's enough to put a relay to the amplifier to disable its output when you want to listen to the mics.
I'm ordering some parts and I'd like to have some advice about which device is best to switch amplified audio signals and microphone signals.
- good old electromechanic relay?
- solid state relay?