I use an Airport Express, connected to my Beosystem 7000, through AUX.
You can either use a minijack to DIN converting cable. (Several available through the web/LifeStyle-AV).

Or you can use the optical digital out (through the same jack). You then need a Toslink Minijack-to-Toslink regular optical cable, and a stand-alone DAC to convert the digital signal to an analogue one that your BeoSound 9000 will understand. This is a more complicated solution, but you can get a better conversion of the sound than you get by using the DAC in the Airport Express.
Your laptop has to be Wi-Fi enabled. But the advantage is that you can use it wherever you wish throughout your home, and still be able to send music to your BeoSound, without having a cable attached. This freedom of movement is yours to have whether you are using a mechanical or optical connection, of course.
If the music on your laptop is compressed with low bitrate MP3, below 320kbps, then I would recommend the cable in the illustration. If you have lossless files on your laptop, then you should consider the stand-alone DAC between the Airport Express and the BeoSound 9000. The Airport Express will transfer files up to and including CD resolution (16 bit/44.1kHz).