In church, we’ve been getting some hum from our computers when we play a music file, CD or DVD and plug it in to our sound desk. It vanishes (read “becomes harder to hear”!) when we crank up the volume, but for quiet tracks, it’s really obvious.
I suspect that it’s a ground loop ocurring between the sound desk and the computer and having done a little Googling today I find that this is a known situation between desks and computer audio sound cards.
So, how to fix it?
The recommended solution is to isolate the audio signal from physically (electrically) transferring from one system to another. 
I’ve found such a device from a JK Audio, the Pureformer Stereo Isolation Transformer. Using a pair of phono cables you connect the output from your sound card to the isolation transformer and then, using another pair, you connect the output from the isolator into your sound desk input.
I’ve found a UK supplier selling it at about £40 + VAT so I’m going to go ahead and get one. I’ll let you know how I get on!
Great, I look forward to hearing the soundtrack without the hum. Mac Man you never cease to amaze me.