Voiceover auditions are not like any other acting audition out there. On the contrary, the voiceover auditions process is completely different. You cannot enter a voiceover audition expecting it to be like a movie audition. The one obvious factor is casting directors are not looking at your face, or movements, but listening to your voice. During a voiceover audition casting directors are looking to see if you understand the character you are auditioning for. Can you imagine Mickey Mouse with a low tone raspy voice? Something like this just will not fit the character and the casting director needs to make sure you are capable of understanding this concept.
Although you may have something prepared for your voiceover audition before hand, the casting director may ask you to tweak your voice a little in a different direction. This particular request is a matter of skills and how flexible you are with your voice. There is not much you can do to prepare for something like this as it usually entails just practicing the versatility of your voice, but you should be aware this is something that may be requested of you. Casting directors are not interested in hearing rehash or regurgitated voices that sound similar to something they've heard before. I'm sure during the audition they've heard many voiceover actors with similar voices.
When preparing for your voiceover audition make sure you bring originality to the table. One way to do this is to perform in front of audience or a group of friends prior to the audition. The last tip for voiceover audition is that you need to be able to take directions in the studio. If the casting director asks you to change your voice a specific way or say something a specific way you have to demonstrate your ability to follow directions. This is really important, as you are not only reading from a script all the time and you have to show you are a dynamic voiceover actor.