The answers are: It does not allow listeners to interpret each character through his or her tone; and it does not allow listeners to review or reread what each character has said.
When hearing the characters voices out loud, and in the hypothetical case that it is a live audition and not a recording, one, as part of the audience, does not have, evidently, the possibility of reviewing or rereading what each character says. This may seem vane, but in reality, it can be very important when reading since sometimes the sense of what´s being read is so profound that, in order to capture in full, one needs to review a certain passage.
Also, hearing the characters has the disadvantage of making their voices concrete and specific according to whoever is speaking. This leaves out the possibility of filling the character´s voice with one´s own imagination, wit, and fantasy, which usually are very important characteristics of a fictional character (literature, in the end, is always a very subjective activity on the side of the reader).
