I have a home wireless network.  I would like to share video and audio files between the computers on the network but am concerned about security risks using this capability.  Is the risk just to the files being shared or are all of the files residing in all computers on the network at risk?

   That all depends on what you make accessible to the network from the computer.  If you are making the entire drive accessible, then the whole drive is at risk.  If you are just making files in the share folder accessible, then just the shared files (and all of the files in the shared folder) are at risk.
   You can reduce this risk by requiring any computer logging on to your wireless network to do so with a password, as with WEP (Wireless encryption) or WPA (Wi-Fi Protected Access) access (this will keep passerbys or neighbors from using your network) and by using a router instead of a hub or access point (this will reduce the risk of someone from the internet getting on your network).  The password in the router should be changed after purchase (see the instructions that come with the router).