Long unbalanced lines are generally a no-no.
“I am in the process of setting up a PC based video conference system where I have a PC (with audio card) and a wireless microphone. For an upcoming job I have to physically locate the wireless receiver some 200′ to 400′ away from the PC. I’m going to run a shielded audio cable between the two but I’m not sure if the system is going to work over the long cable length involved.”
It probably isn’t going to work over that long of a line. First, you are going to have a ground loop between the two devices because there is no way the two are going to be plugged into the same electrical circuit (a sure way to build a ground loop). Second, an unbalanced line of that length is likely to pick up a lot of garbage (EMI and/or RFI) along the way even if it isn’t part of a ground loop.
The most foolproof solution is transformers. Transformers will let you electrically isolate the two ends of the unbalanced line from one another, which is a sure way to prevent ground loops (assuming you don’t otherwise tie their signal grounds together). And while you’re at it you may as well use balancing transformers. You can balance the signal at the source (your wireless receiver’s output) and then unbalance it again at the destination (your PC audio card). This ensures that the long length of the line is balanced and you can benefit from common mode rejection at the destination, which will cause most of the noise picked up along the way to be cancelled. When you do all of this just make sure you don’t tie the grounds or the shields together. This will make a ground loop and cause a hum. Only attach shield at one end.
While this is a somewhat specific and uncommon example, these principles hold true for all kinds of unbalanced lines. You can’t just lift the ground of an unbalanced line (in most circumstances) because the ground is part of the signal path. But if you balance it you can lift the ground and benefit from the noise cancellation of balanced lines as well.