Testosterone therapy given to people transitioning into male will have a direct effect on the vocal cords. It will make the vocal cords thicker. Thicker vocal cords in turn, produce a deeper / lower pitch.
Although for most people, testosterone can have quite a strong effect, there are people for whom it may only produce a mild effect. It’s rare that testosterone has no effect but it can often be less of an effect than an individual might hope for.
Traditionally not many trans males came to receive voice therapy because it was thought that the testosterone would do everything they needed. However as the number of individuals transitioning into male has grown, those higher numbers have also brought with it the realisation from some individuals that they are not happy with their voice. They may think for example that their voice has not gone low enough and that they would like it to go lower.
Of course, a lower pitch in males is only one aspect of the voice and communication. There are several other aspects such as intonation and resonance that can sometimes benefit from assistance in terms of seeing a specialist to help develop those aspects further. Also, testosterone will not necessarily make one’s voice any louder and so some professional guidance might be useful in order to produce a “bigger” voice from smaller structures.
Primarily, testosterone will change / lower the pitch but it may not necessarily make a person feel 100% happy with their voice. Conversely however there will be some who go onto testosterone, be blown away by what happens to their voice and never come and see a speech therapist, and so the outcomes must be treated on a case-by-case basis.