Open a track, tap Mark at the start and again at the end of each phrase, then loop a phrase and sing it back in the gap. The file stays on this device. Nothing is uploaded.
Phrases sync through a secret gist on your GitHub account. Create a classic token with only the gist scope (GitHub → Settings → Developer settings → Personal access tokens → Tokens (classic)), paste it here, and do the same on each device.
Keys: Space play, ← → phrase, R restart, M mark start/end, Esc cancel, B back 3 s, L loop, − = speed.