tl;dr: OnKey is a free, open-source singing trainer for Apple Silicon Macs that listens to music playing in Spotify, Apple Music, or YouTube and overlays your pitch against the vocal melody in real time. On first play it learns the song locally (using CREPE, GAME, wav2vec 2.0, and MDX-Net for vocal separation and pitch tracking), then scores subsequent attempts line by line. Audio is processed in-memory with no accounts, analytics, or music storage—only Shazam signatures and lyric metadata are sent out, and both can be disabled.