I use the Logitech Z-680 speakers, is there a way to mix 5.1 surround sound in FCP 4.5?

Not with FCP alone that I am aware of but you certainly can within FCP have 6 tracks of audio edited any way you see fit, export them with Compressor and then encode that into Dolby 5.1 with the A.pack CODEC included with DVDPRO. I have done this once with 4 channel sound recoreded with an XL1s, it does work and it is pretty neat.

A.J.