When you have finished this first stage of the discussion you should have arrived at a consensus on a preliminary definition. Music is sound that is pleasing to the person who hears it. You should draw out of them the fact that not all pleasing sounds are music. But those sounds are not music.
Some bird calls are beautiful sounds.
But most bird calls are not music. Draw more examples from the students of sounds that are pleasing, but are definitely not music. As you are doing this make note that sounds that are pleasing to some students are not pleasing to others. Make a point to make these observations out loud so that the whole class hears. You will use this below.