Music plays a big part in Resident Evil, with many musical puzzles.
The most famous piece of music in the series is the Moonlight Sonata by Ludwig van Beethoven.
It's official title is The Piano Sonata No. 14 in C-sharp minor "Quasi una fantasia", Op. 27, No. 2.
The first movement of the piece is the key to opening a hidden section of wall.
In Resident Evil 1 either Jill will play this on the piano in the bar or Chris will leave Rebecca to practice the piece.
This is Jill in the original game.
Rebecca after a lot of practising in the remake of Resident Evil.
If you'd like to play it yourself, it's widely available in sheet music form. You can also see it online
in various places, including here although I'm not sure if it's accurate as I'm not a pianist.
Elsewhere there are musical puzzles in Resident Evil 3 where you have to set up a music box
to play the correct tune, in Code Veronica where the music boxes in the Ashford's private residence
have multiple uses, and a piano roll which you can use in a piano in the games (casino) room.