Music that's pleasant to listen to is tuneful. A tuneful lullaby will put your little sister right to sleep.
If you whistle melodically, in a way that sounds lovely to people who hear it, your whistle is tuneful. You might describe a new album from your favorite band as being full of tuneful songs, or the birds in your yard as making tuneful chirps and tweets. Tuneful comes from tune, which evolved out of tone and its Proto-Indo-European root, ten-, "to stretch."