Q: Airport Express Devices?

Sorry I may not have explained it corrrectly. I had a pair of logitech speakers hooked into the speaker jack of my Imac, all the sounds from the mac were rich. I wanted the same sounds out of my other devices, so I bought the express and hooked the speakers into them, The musi sounds rich across all my devices. The issue now is that all the other sounds (internet video for example) comes through the internal speakers (nt great sound.). Is there a way to get the Imac to use the express for the sound (other than just Itunes).

I can't find any setting that has the mac see the speakers (say as a/or connected to a wireless device) and use them