Etude · Looper - transpose, slow down, loop by Onur Celep

Etude adds a musician's practice controls to online video.

Transpose to any key and fine-tune pitch in cents.
Slow down (or speed up) while keeping the pitch, so you can drill a fast passage.
Loop an A-B section and save named loops per video.
Audio is processed locally in your browser. No accounts, no tracking, nothing is uploaded. Works on YouTube and other standard HTML5 video.

Open your own local files instead? Use the Etude Looper web app: https://onurcelep.github.io/etude/looper/