Symphony is a music based vertically-scrolling shoot 'em up where the player has to liberate his music collection which is under attack by five demons. The player can import his own songs into the game and each song becomes a unique level. As the music gets more intense the enemies get more aggressive. The action is viewed from a tilted top-down perspective from which the player can move his ship freely with the mouse. Enemies enter the playfield from the edges of the screen or from above and will then absorb and corrupt the music. The player has to liberate the music by shooting the enemies who then drop notes called inspiration which the player can collect. The player has infinite lives but when the ship is destroyed some inspiration is lost. Collecting inspiration repairs the ship. Liberating whole waves of enemies leads to chain bonuses being earned.
The goal is to collect inspiration and reach the song's score targets. When score targets are reached the player earns kudos. Inspiration and kudos are used to unlock and upgrade items. Each song has its own unique item that is discovered when a song is completed. There are two types of items: those that can be equipped as weapons on the ship and those that appear on the playfield and has to be collected. Four items can be equipped on the ship and they can be mixed and matched freely. Each item can be rotated (which allows shooting in multiple directions) and it's possible to specify which mouse button controls each item.
There are six difficulty levels and they are unlocked by defeating demons. Demons act as bosses and each demon contains a piece of the Symphony of Souls. By defeating three demons one page of the Symphony of Souls is retrieved and the player unlocks the next difficulty. When playing at a higher difficulty more kudos can be earned.