I am right in the middle of doing this job now.
I am going to cover the floor in sound proofing felt from...
http://www.frost.co.uk/item_Detail.a...0Coated%20Felt
This is also avalible from many other suppliers.
I am going to cover the floor with this material as it is about 10mm thick but leave a channel cut out for the wires to run. Once all this is covered with the floor carpet the wires will be well protected and you will not see a line running through the carpet from the cable underneath.
Craig.