LRClib. They have an official GUI app that downloads a sidecar with plain or synced lyrics depending on availability an settings. For it to work reliably, you need to tag your library reasonably well with something like Picard. And your music client obviously need to support lrc-files.

This is not a wikipedia page. It is on the domain ‘consumerrights.wiki’. It is a relatively recent initiative created for crowd sourcing reports on poor products and companies that act in an anti consumer fashion.