Thought-speak is the way Andalites communicate since they have no mouths. It is a language that almost all species can understand. It is not quite telepathy, as beings using thought-speak communicate through universal mind symbols rather than actual thoughts; however, it is like telepathy in that there is no sound being transmitted or received. There are two different types of thought-speak: open and closed. Open thought-speak is where you broadcast what you're thinking to everyone, while closed thought-speak is where you communicate to one person or a selected few.
In The Suspicion, Cassie recognizes Ax's thought-speak voice, "which meant he was probably in his normal body." This suggests that you may sound different in different morphs and everyone's thought-speak voice is as distinguishable or more from a sound producing voice. Many other aliens communicate by thought-speak. The Animorphs can also communicate in thought-speak, but only in a morph as shown in a few books where Ax has talked to the others via thought speak, although he was human at the time. There are also limits on thought-speak, such as distance. (Unless using a Mirrorwave call.) For example, in The Visitor, while Tobias was on the roof of Chapman's house, Rachel did not hear his Thought-speak in the basement. In The Underground, Rachel couldn't Thought-speak past an estimated 150 feet underground as a mole. In The Other, Gafinilan-Estrif-Valad states that he can use thought speak with his since-childhood friend as long as they are on the same planet, though the exact words are unclear the "sound" helps him know Mertil-Iscar-Elmand is living.
Use of thought-speech
Thought-speak is used by those in morph, contemporary Andalites, Leerans, Iskoort, Mercora, the Helmacron, Garatrons (who have an extremely fast thought-speak), the Ellimist (as an Andalite), and the Crayak.