Why did the writer and his friends start to laugh when Professor Herr Slossenn Boschen sang his great tragic song, and how did the professor react to their response?

Two German guests, whom everyone was mostly ignoring, interjected to insist that a colleague of theirs could sing the funniest German songs they had ever heard. They offered to fetch him, and the man soon arrived to play. Though it turned out that his song was actually tragic, J. and the other guests laughed constantly, thinking it polite to do so. However, they actually angered the pianist, and the two German liars escaped before the song was finished, having played their practical joke.