What can be done about an unintentionally disruptive classmate? I have a classmate who frequently disrupts class by asking nonsensical or unrelated questions, or by making nonsensical or unrelated interjections to the professor. He is autistic, and all the professors I have shared with this classmate (he and I are the same year in the same field of study) have followed the same pattern of humoring him to an extent by attempting to answer his questions, or by thanking him for his interjections, but eventually just ignoring him. I'm sure he has good intent (thinks his questions and comments are relevant or useful) but he doesn't seem to catch on when the professors no longer reply to him (and all of his classmates are rolling their eyes and shaking their heads).
Today in lecture, he sat at the far back of the auditorium (there were tens of empty rows in front of him) and then asked the professor to speak louder. The professor responded by asking my classmate to move closer, as he could not be loud enough to be heard in the very back. My classmate replied by stating that he was comfortable, and then asked a series of questions about whether the lecture material was available online, in the textbook, etc., all of which had been covered in previous lectures anyway, and were on the class website and syllabus.
It has become apparent (after three years of being with this classmate) that the department and/or professors do not have any intent of trying to encourage acceptable class behavior with my classmate. Is it safe to assume that they are aware of his disruptive behavior, and are simply not addressing it due to his mental condition, or should I bring it up with the professors or the department? Would such a report be frowned upon as insensitive due to the classmate's mental condition?
Edit 14 January 2017
In regards to the linked question, the student is not actually being offensive, just disruptive. Being offensive (specifically in regards to racial slurs) would merit additional discipline.
It sounds like everyone who's dealt with it decided the student's behavior was a minor annoyance at most, or even that their questions were valid and worth answering. Repeating material that was covered in previous lectures, in the syllabus, or on the website is...well, part of the job, and many professors explicitly welcome questions about past material or any general course questions of that nature. It is generally assumed that students aren't tape recorders with perfect memory, and if a professor is really bothered by it they are free to answer with "that's covered in the syllabus". Off-topic questions are typically easily handled with "that's beyond the scope of this course, so questions like that would need to be asked in office hours/email/someone else." If the professor goes off on 30-minute tangents about meatballs because a student asked a question about them that has nothing to do with the course, well then you'd have a valid issue to raise with the professor.

Based on your descriptions, all the past professors found minimally disruptive ways of handling less-than-perfect questions - such as limited answering, ignoring comments that did not need a response, etc - and found a way to move past it. If there is regular behavior by another student that rises to the level of such a disruption that you feel you are materially harmed in your education by being subject to it, by all means bring it up to the appropriate faculty/administrative contact.

However, I must point out that there is a baseline reality that standard deviation in the human population is greater than 0; people are not all the same.  This means that your standard of behavior will not be followed by everyone, and others will have different standards of behavior than you do. It often pays to first ask yourself: "is this so far out of line that it I really need it to be dealt with?" If it's just an annoyance, or something that really isn't really that big of a deal, empathy and patience might be a more useful strategy than complaint. 

As to how a potential complaint would be received, that rather depends on what you are complaining about. If you complain that a fellow student asks questions about the class that are already covered in the course materials, some professors will agree (they wish students read the syllabus too), and some will think "that's a weird complaint, I wish more students asked, because it sure isn't like the ones that don't ask actually read it all". If you complain that the student is a bit odd, well, yes, they probably got that idea by now. If you complained that a student was, as an example, yelling racial slurs in the middle of class, then that would certainly be seen as a valid complaint, while another complaint about a student with tremors shaking and you find that distracting...not so much.

So in the end, I would advise you to consider the seriousness of your complaint. If you feel that what you've witnessed/experienced is materially harmful and wish to make a complaint, make sure you specifically address exactly what is the issue and what you would like done about it, such as: "a student regularly asks questions that are not pertinent to the course, and the professor allows the distractions to cut into class timer, or ignores continued talking and comments instead of telling the student to stop."