I remember from the book where Tobias gets tortured, when he morphs Ax, he feels that the Andalite mind is naturally full of optimism. During the series, many Andalites are battle-worn and wary. Thus, (I feel like I'm writing a speech) the Andalite's thoughtspeak would would grow from an optimistic child to a hardcore warrior, although in book 54, Marco says that civilian Andalites are much mellower than their warrior counterparts. So civilian andalites might have different thoughtspeak voices than warriors. Plus, I think the reason that the Animorphs could understand each other in thoughtspeak, is because a human naturally adopt their human voices, since they've heard it the most (avid television watchers exempted).