A proper noun is a noun used to refer to a specific person, place, or thing. For example, the names "John Smith," "London," and "the Eiffel Tower" are all proper nouns.
A common noun is a noun that refers to a general category of people, places, or things. For example, the words "man," "city," and "building" are all common nouns.
The names of music are common nouns because they refer to a general category of things: musical compositions. For example, the names "The Star-Spangled Banner," "Strawberry Fields Forever," and "Für Elise" are all common nouns.