Transfer Audio output of my MacBook wirelessly

If you have a newer Mac with Mountain Lion you can also use it's built in AirPlay functionality.

Supported models as announced by Apple.

AirPlay Mirroring requires a second-generation Apple TV or later, and is supported on the following Mac models: iMac (Mid 2011 or newer), Mac mini (Mid 2011 or newer), MacBook Air (Mid 2011 or newer), and MacBook Pro (Early 2011 or newer).

Go to System Preferences->Sound and select Apple TV from the Output tab. This will play all system sound from your Mac.

You can probably do this on per App basis also, but I haven't done this my self. You can find an article about this from here: http://joris.kluivers.nl/blog/2012/07/25/per-application-airplay-in-mountain-lion/

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 

Airfoil is probably your best bet as far as software goes. It's an app that captures any sound from your machine and beams it to a variety of devices: iOS devices, other Macs, and Airplay devices such as an Apple TV or Airport Express.

As for the hardware, some hi-fi systems are starting to build in Airplay (which is the new wireless audio protocol Apple has introduce). If you don't want to buy a whole new stereo, I'd buy an Apple TV. $99 bucks for wireless audio and video is a pretty good bargain in my book. And, more and more apps and devices will take advantage of Airplay (including video Airplay) as iOS 5 rolls around.

You could also use an Airport Express, but that would limit you to audio only.