Your use of 'Sir' sounds strange to me, but maybe it works in your part of the world: auditors are in a position of authority, and I would be surprised to hear them kowtowing like this. But maybe it's standard in your country - with local auditors and English native-speaker teachers.
I think this is a cultural thing. I know it mostly as an Americanism, but it's perhaps used elsewhere in the world too. In this context "auditing a class" doesn't mean what it sounds to English ears like it should!
It means attending a lecture series (and possibly doing the course assignments too) but not getting graded for it, and not having it count towards your final degree. As such an auditor in this sense would be just another student, and would in most places use an honorific when talking to a professor or lecturer I think.
EDIT: @ birdeen's call - I wouldn't have called it kow-towing either, but it
is a clear signifier of the relative status of the two people. Using a "Sir" tends to give the idea that you are offering respect to or acknowledging the superiority of the person you are addressing. As such, an auditor in the English sense would be superior to a teacher, if they were auditing (inspecting) their teaching proficiency, and would be unlikely to use "Sir". But in the American sense where an auditor is a student, they are naturally more likely to be showing respect and using "Sir". Kow-tow is perhaps a very negative word for that.
