You may find you get the following problem when trying to stream music to multiple speakers. Streaming to either local computer OR your airport express works, but when you try to stream both, you get an error message that looks like this:
“An error occurred while connecting to the remote speaker ‘speaker_name’. An unknown error occurred (-3256).”
This is most likely caused by a firewall issue. If you’re running Tiger, then there’s a fix here. If, however, you’re running Leopard, then it’s a little different:
1. Open System Preferences
2. Click Security
3. Click Firewall
a. “Allow all incoming connections” – but I don’t recommend this, so
b. “Set access for specific services and applications”. Click the + symbol, then find iTunes in your Applications folder. Then set its permission to Allow incoming connections.
You should now be able to stream to multiple sets of speakers. Whilst you’re playing about in the firewall settings, click Advanced and enable the stealth mode, which seems to be off by default. You should also check that logging is enabled, as it’s possible that the change to your firewall settings will interfere with other applications you have installed.