Salamander Grand Piano
The grand piano sound is the Salamander Grand Piano V3 by Alexander Holm, recorded from a Yamaha C5 and released under the Creative Commons Attribution 3.0 Unported license (creativecommons.org/licenses/by/3.0). Source: archive.org/details/SalamanderGrandPianoV3.
The site uses the compressed MP3 versions published by the Tone.js Piano project (github.com/tambien/Piano), four of the sixteen velocity layers, 30 notes each. The files are served as published; during playback your browser trims the silence at the start of each recording, shortens its tail with a fade, and shifts its pitch by up to one semitone to reach the notes in between. Those changes happen only on your device.
Synthesized sounds
The electric piano and organ sounds are generated in the browser by the site's own code and are not based on any recording.
Music symbols
The sheet music is drawn by the site's own code when the pages are built. The music symbols in it (clefs, note heads, flags, rests, accidentals and time-signature digits) are outlines from the Bravura font by Steinberg Media Technologies GmbH, released under the SIL Open Font License, Version 1.1 (openfontlicense.org). Source: github.com/steinbergmedia/bravura.
Songs
All songs are public-domain melodies (traditional tunes and works by composers who died long ago) in simple one-hand arrangements written for this site.