How does voice affect characterization?
The voice affects characterization because it lets you into the character's head.
The human mind can tell majority of people apart just by listening to their voice. The way they speak, their tone of voice, their vocabulary, they all let you into the character's mind.
For example, a bratty teen (totally not me) could be expressed with a nasally, high-pitched tone of voice. If this bratty teen isn't supposed to have a very high IQ, you can subtly express this by using simple words and abbreviations (e.g. whatever, as if, ugly as, etc.) This is far more effective than spelling out, 'she doesn't have a very high IQ'. Readers prefer to figure out the answer by themselves, rather than having everything being spelled out for them.
Hope this helps :)