Go on a sound hunt with your child. When you hear a sound, but cannot see the source, try to guess what's making the sound. Then track the sound down. Were you right?
Start inside the house, listening for common household noises. Make a guess about what's making the sound and then track it down to check.
Once you've played inside, move outside. Listen for a sound you can hear in your neighborhood, predict what is making the sound and try to follow it to find out if you are right.
For a great extension activity, tape the sounds you hear on a tape recorder. Play them back and try to identify them.