The comma is mandatory. It separates the main clause (‘I know’) from its subordinate clause (‘where the plate is’). It's a rule in written Russian.
In casual speech, the pause at that point is not so long as in the audio.
Disclaimer: I'm neither an expert nor a native speaker, only a fellow learner!
I had the same question and I commented above and got an answer like: "By the pronunciation you must know it". This answer didn't tell me anything, but I compared with my native language that is Spanish. We can say the same word order for a question or answer. For example: "Yo sí voy!" (I'm going!) And, "Yo sí voy?" (Am I going?). The difference is that in the question you must take the last part (vowel) and make it sound larger and higher. And for the answer on the other hand make the same but not in the last part. So in my example would be: "Yo sí vooooOOY?"(notice that capital letters are louder) For questions is: "Yo SIII VOy.." (Sound is lowered at the end) But here in Russian I can't get the difference between question or answer . Hopefully it'll help.
You'll notice raised pitch in the word in question. The pitch is raised in the stressed vowel. E. G. это наши тарелки - these are our plates. Clear declartion. Now if we want to ask if the the plates are ours. (are these our plates?) we keep the sentence intact but add higher pitch to the word we are asking about. In this case we want to know if they are ours so pitch up наши. "A" is the stressed vowel here so pitch up there. If for some reason you don't know if they even are plates you could add the higher pitch to тарелки. Hope this clears some of it.