That is scary if you applied based on marriage, rather than PR. I think most people applying based on marriage understand that they are not required to sing the anthems. Had that happened back during my interview, I would be screwed as I can't sing either and even if I knew the words, my singing would have been a pain to the ears of all who had to listen. On the other hand, it is my understanding that if you are applying based on PR, then whether or not you are married to a Thai, you will be required to sing the anthems.