You should've played the entire game. There are credits at the end. I can make the job a little easier for you here:
Graphics:
- Sky Background: Pauilina Riva https://opengameart.org/content/sky-background
Music:
- 8Bit Title Screen: Joth https://opengameart.org/content/8bit-title-screen
- Happy Tune: Syncopika https://opengameart.org/content/happy-tune
- Boss Battle #3 [8bit]: nene https://opengameart.org/content/boss-battle-3-8bit
- Sunday at the disco: Snabisch https://opengameart.org/content/sunday-at-the-disco-nes-version
Sound Effects:
-The Essential Retro Video Game Sound Effect Collection: SubspaceAudio https://opengameart.org/content/512-sound-effects-8-bit-style
-Victory: celestialghost8 https://opengameart.org/content/victory