Help! My Logitech Z4 speakers have just stopped working on my iMac and I can't figure out why. I've checked all the cables and they're all where they should be, the only sound I'm getting now is from the iMac's own speakers. Any ideas what might have happened or how I can fix this? I haven't added any other hardware to the iMac recently and I can't think of anything else I might have done that has caused this. Help! Thanks.