That's a tad complicated, actually... but mainly because characters don't have an "IfUsed" field in their properties.
What you can do to get around that is one of two things:
1) Have the character's script hard-coded to set it's target to the other entity. (Using the entity's name) Then activate it's target.
2) Have your character activate nearby entities with a certain value. (Not sure what this command is, but I know it was used in stock scripts... not coming to me off the top of my head) Then have your "destination object" at the end of the waypoints wait for an activation from the character. Then your object (which DOES have an IfUsed field) can activate the light.
After that, it's just a matter of having the light play the sound you're looking for.
The one and only,