How to run wires through a wall with existing wall wiring? (Tight Spaces)
their are times I will cut a hole for an old work box, drill from the attic down or from the crawl space up (attic down is easier as you can see the 2x4) drop a string with a small 2oz fishing weight,  hook wire on and pull up, then do the same at the location of your sound system , install an old work box in each location these are the boxes that have wings that pull them tight to the sheetrock, or a low voltage ring that snaps in place and you fold the metal tabs around the sheetrock, then put a coax cover plate and it looks very profesional.