digital sound output on MBA ? toslink adapter ?

Yep. You just need a toslink to mini-toslink cable.
http://www.monoprice.com/Search/Index?keyword=Toslink+to+mini

The mini-toslink looks like a standard headphone plug. Just a bit longer.
All most macs for quite a while have had a combined optical/analog port ( with the possible exception of some mac pros, and the air. You'll have to check if you own one of those)

Edit ----
I didn't realize the MBair didn't have optical out, changed my post.
Sorry for the misinformation.

 

Yes, Toslink/Mini Fiber Optic Cable

Yes, I just set-up an apple airport express (AAE) into my 2 channel stereo system. All you need is a Toslink/Mini Fiber Optic Cable. I used this cable from the AAE digital output into my DAC which runs into my pre amp. This allows me to stream iTunes into my 2 channel stereo and can control it on the couch via my iPad wirelessly!

Awesome and id sounds great!

RonW.

 

The Macbook Air does not have digital audio out via toslink interface, only by the Mini Display port.

 

Indeed. If you want digital out either use HDMI or an external USB soundcard that does have digital out. The MBA only comes with analog out but its port is both audio in as well as out (which is why the Apple headset with mic is fully functional).