Sound-alike casting - I've seen a lot of it recently and in my opinion, there are two types of sounds-alikes...
1) Dead on match. They don't want you to add your personality or to see what you would do with the role. Your task is solely to match the vocal quality, cadence, and personality that already exists. This may come into play if the person playing the role is unable to come in for pick ups for whatever reason. Basically your voice will be mixed in with the other voice and they want it to be seamless.
2) A vocal match, but to add your flair. They want someone to have the same sound as the previous actor playing the role, but are open to you making it your own. For example, a movie becomes a TV show and not all the original actors sign on. The casting directors may look for someone to have the same sound, but since the role will be yours for 52 episodes (fingers crossed) they want to see where you would take the character.
So when you get these types of auditions, take a look and try to get a feel for which one they are asking you to do.