i) A combination of musical melody, and sung lyrics.
Modern songs often have acompanying harmonies and complicated rhythms. Songs from longer ago are very simple in their accompaniment, and a lot don't have any.
ii) It can also mean how a man hears a woman's voice, when he is in love with her. This, however, is quite an old saying.
i) I sang a song
ii) Her song was sweet to my ears
A song is a musical piece that is usually around 3 minutes long if it is popular music. A song could also be something like campfire songs which are not 3 minutes long. They are usually longer.