For quick prototypes that need to feel good fast, here are free effects and UI with matching sounds:

• Pixel Combat VFX Lite + SFX Lite: explosions, hits and slashes with matching sounds
  https://willosuna.itch.io/pixel-combat-vfx-lite
• Pixel Pickups VFX Lite + SFX Lite: coins, gems, hearts and loot with matching sounds
  https://willosuna.itch.io/pixel-pickups-sfx-lite
• Magic Spells VFX Lite + SFX Lite: pixel art spell effects in 10 elements with matching sounds
  https://willosuna.itch.io/magic-spells-vfx-lite
• Retro UI Kit Lite: arcade pixel art panels, buttons and bars
  https://willosuna.itch.io/retro-ui-kit-lite
• Essential UI SFX: 150 UI sounds (click, confirm, error, notify, coin, purchase...), 5 versions of each
  https://willosuna.itch.io/essential-ui-sfx

Sounds come as WAV + OGG, graphics as transparent PNG. I made all of these myself, drawn and synthesized from code: no third-party art or samples. Free for commercial use; a credit to Willosuna is appreciated.

Good luck with the jam!