I've pondered this, too. It would be pretty cool if it worked that way, wouldn't it? I know I would have fun with it.
In HBC, Aldrea is apparently able to imitate Alloran's voice, but given that the computer asked for thought-speak identification, it seems to be the assumption that the voice came with the morph, and that Andalites cannot normally imitate each others' voices. It would be pretty silly to have that sort of identification if any Andalite could trick it.
Also, in #17 (I believe that was the one, anyway), Marco does a Schwarzenegger (sp?) impression. If I remember correctly, his impersonation is described as "decent" rather than "perfect." Now, if he could do a perfect imitation of the Governator just by thinking about it, why didn't he?
So, all in all, evidence points to each person having a unique thought-speak voice that they can't really alter too much, at least not any more than you can alter your own voice. And this makes sense to me, too. The fact that they can all recognize each others' thought-speak voices is pretty telling. I don't know about anyone else, but when I think to myself, the voice that narrarates my own thoughts is rarely (if ever) my own. So if they all just spoke in whatever voice they happened to be thinking in at the time, nobody would ever be able to tell who was talking.
As for Ax imitating V3 in #7, that may have been simply because those controllers had not heard or seen V3 before. I mean, it became very obvious once the real V3 showed up, which one he was.