Simplify the following text for a Grade 7 native English-speaking person.
Original: Sheet music usually refers to a `` single sheet '' of music ; that is , one song or piece printed separately .
Simplified: Modern sheet music may come in different formats . If a piece is composed for just one instrument or voice ( such as a piece for a solo instrument or for a cappella solo voice ) , the whole work may be written or printed as one piece of sheet music .