The physics is the sequencer — every bounce is a note, always in key.
COOP/COEP headers, so the browser will not grant it shared memory and audio cannot run on its own thread. Serve it with Cross-Origin-Opener-Policy: same-origin and Cross-Origin-Embedder-Policy: require-corp.Drag on the board to draw a line. Balls bounce off it and it plays.
Everything else is behind ?.
Everything the board can do to itself.
Three sentences, then the shortcuts.