One of my favorite tools is a 2"x3" mirror on an adjustable handle and SMALL flashlight on a chain. While it can be a bit of a pain to get them to play nicely together, you can secure the flashlight with the chain and drop it into the hole so it points down and fills the space with light. Then use the mirror to look down into the hole. Both items are available from your local hardware store and can let you know exactly what is going on in the wall with your wire.
One of the situations that's possible is that there is some sort of fire break or extra stud running across the wall halfway up - or a couple of feet up, and if you cut a hole under that stud, you will then need to cut ANOTHER hole above it to get where you need to be.
On the other hand, if you are using surround speakers you might be able to enlarge the hole that the wire fell down into. If you don't own the speakers yet, consider getting in-wall surrounds and just cutting out a large opening in the wall which can accommodate in-wall speakers and can give you enough room to get your entire arm into and really work in.
These then, might become VERY handy...
http://cgi.ebay.com/11-Electric-Fiberglass-Wire-Push-Pull-Rods-Fish-Tape_W0QQitemZ310155689624QQcmdZViewItemQQptZLH_DefaultDomain_0?hash=item4836b83e98&_trksid=p3286.c0.m14
It's a question of how much of a rush you are in and if you want to deal with additional drywall damage. I've done it both ways - I've spent the time fishing and hooking and dealing with clean lines, as well, I've opened up walls. At the end of the day, fishing the wire was a bigger pain to do, but had far less overall work involved in it and I wasn't worried about whether the paint would match perfectly what was already there.
Good luck!