Transfer Audio output of my MacBook wirelessly
I now found a good solution. The Belkin Bluetooth Receiver. It only costs 30$. Way cheaper than the AirPort+Airfoil Solution. AND it only has 100-400ms latency. Cheaper, faster and better sound.

But one final note:

You have to change the bluetooth settings a bit to make it sound perfect. Disable bluetooth, add the following lines and reboot.

defaults write com.apple.BluetoothAudioAgent "Apple Bitpool Max (editable)" 80 
defaults write com.apple.BluetoothAudioAgent "Apple Bitpool Min (editable)" 48 
defaults write com.apple.BluetoothAudioAgent "Apple Initial Bitpool (editable)" 40 
defaults write com.apple.BluetoothAudioAgent "Negotiated Bitpool" 58 
defaults write com.apple.BluetoothAudioAgent "Negotiated Bitpool Max" 58 
defaults write com.apple.BluetoothAudioAgent "Negotiated Bitpool Min" 48