But thought speech isn't technically audible, so it shouldn't be based on your physical voice at all. My best guess is that the projected voice is determined by how you hear your own voice.
A weak analogy, for those who have seen The Matrix, would be how they described how clothing and hairstyles are generated upon entering.
Still, I see no reason why one wouldn't be able to use thought-speech to project any communication based thought at all, whether or not it can be done in a voice. Andalites do not have a physical voice, so technically their language could use completely abstract sounds if reproduced, such as beeps. If you think about a bell ringing, there isn't any logical reason why you couldn't be able to send it to others. It's not like sending specific memories or even visuals; its just a sound. Think about the African languages that use clicks. Just because they don't involve vocal cords like western languages doesn't mean it couldn't be thought-spoken.