QuickClef stands on generous open work. Thank you to everyone below.
The piano voice uses samples from the Salamander Grand Piano by Alexander Holm, licensed under Creative Commons Attribution 3.0.
Band and orchestral instrument sounds come from the FluidR3 General MIDI soundfont by Frank Wen, distributed under the MIT license.
Music engraving is rendered with VexFlow (MIT license) using the Bravura music font by Steinberg Media Technologies, licensed under the SIL Open Font License 1.1.
Display typography uses Barlow Condensed by Jeremy Tribby, SIL Open Font License 1.1.
Audio scheduling and synthesis run on Tone.js (MIT license).