Translation:You like to listen to your son play the flute.
The finite verb is piace. Everthing else is an infinitive. In English we say "he plays the flute" but "I like to hear him play the flute." You could just as well ask "why isn't it 'plays'?" The answer is the same, although non-linguists don't usually call it an infinitive without the "to" in front of it.