Instrument sounds load the first time you pick them (about 1–3 MB each) and are then saved for offline use. Grand Piano: Salamander Grand Piano by Alexander Holm. Other instruments: FluidR3 GM soundfont by Frank Wen, rendered by Benjamin Gleitzman. Both are licensed CC BY 3.0.

Tip: on iPad, tap Share → Add to Home Screen. On Android, use the browser menu → Install app. On a computer you can play with the keyboard: Z–M for the lower octave, Q–U for the upper one.