If the chazzan sings the kaddish that is between פסוקי דזמרא and ברכת ק”ש, and you can’t hear the words because of the singing, can you answer if you hear congregation answering?
If you know where he is up to you can answer kaddish. There was once a massive shul in Alexandria Egypt that was so large that the people couldn’t hear the chazzan. In order for the people to answer kaddish etc. a man would stand near the chazzan with a cloth and lift the cloth when it was time to answer something. The people would follow along, so they knew where he was up to and answer that way. It is definitely better to hear the word from the chazzan, however if you didn’t you may still answer.
Succah 51b, Rema O:CH 124-11, M:B 124-46.