@readersRequiem: You are clearly either forgetting or unaware that all the times she spoke more than just a handful of stuttered, barely understandable words were under very specific circumstances.
[ol]
[*] She was talking to only one person.
[*] Said person was someone that she has grown quite familiar with over the course of at least a single semester's worth of time, i.e. a "best friend".
[*] Nobody else was around in the vicinity, whether the two were in an open street or in the privacy of a stairwell. Alternatively, there's enough distance between the two and the people nearest to them as well as enough ambient noise and commotion that Komi knows for certain that nobody can overhear or is even paying much attention to them.
[/ol]
Any time you see one or more of those above conditions compromised, you'll see that Komi finds it much harder to muster the courage to talk, resulting and shorter and more stuttered sentences, until she reaches the point where she finds it
impossible to talk.
Fun fact: The first time this had been brought up ages go, it was remarked that this actually a very realistic depiction of social anxiety, giving the impression that Oda is either well-versed in the subject or is in contact with someone who does.