Switch Optical Audio without disconnecting cable

Hi All,

I have recently connected my iMac to my surround sound system using an optical cable and just wondered if it was possible to switch between using my surround sound and the internal speakers without having to disconnect the optical cable?

I know for the standard audio jacks you can use something like the Griffin iMic but are there any optical alternatives? I would prefere to just it just using software but if necessary I suppose I could go for a hardware solution.

Thanks,

Mike.