Like so much of English, what you say here is determined not strictly by grammar but by idiom and usage.
Did you hear what I just said? is idiomatic, and depending on tone of voice could be made to sound perfectly polite. But it has the potential to be brusque and offensive, if said without care.
Did you hear what I say? is neither grammatical, nor idiomatic. But you could say Do you hear what I say?, but this will almost always sound impolite.
Both of the above could carry the implication that the person was not listening when they should have been.
If you are genuinely concerned as to whether someone heard you or not, more politely you might say Could you hear me just now?, or even Could you hear what I said just now?
If your concerns are wider than this, involving use of different tenses in the same sentence - please elaborate. But there are many previous questions on the site about that.