I really like the sound effects and music. One suggestion is to make bigger boards and also allow one to place multiple items in the same square. Many online sudoku games allow you to write multiple numbers in the same square.
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Thanks for the feedback, the sfx and music used are: https://assetstore.unity.com/packages/audio/sound-fx/universal-sound-fx-17256 and https://assetstore.unity.com/packages/audio/music/orchestral/ultimate-game-music..., you can check them out if you are interested, really good collections. I'll add the note function to TODO for phase 2.