Is it analog or optical? Analog, i'd say you will have to connect to your computer directly. If it optical, you need a Thunderbolt to optical if such a thing existed. Which i doubt does.
So, i sugget your go the fun route. Get yourself an airport express, it'll turn your Bose speakers into Airplay speakers. You can set them anywhere you want in the room or house. With ML you can select them as the default device for all system sound output. And if you have an apple tv you can stream right from it to the speakers.
If you can find a previous gen AE on Apple refurbs or amazon, get it. Its a much more convenient form factor than the new generation.