Sound

Use speakers, not headphones.

How long sound takes to go out of the speaker and back in through the microphone: the delay you'd correct for when recording along to playback. A burst of noise (a maximum-length sequence) plays, and the recording is searched for it by cross-correlation, to a fraction of a sample. Each repeat of the sequence is one measurement, and runs add up, so the spread shows how steady the audio path is. The browser's own figure covers only the output side.

Keep still and quiet while it plays.

What the room does to a click. A burst of noise plays and the recording is matched against it, giving the sound arriving directly, then reflections off walls, floor, ceiling and furniture, then a fading tail. Each reflection's delay is extra distance travelled; with the speaker and microphone together, the surface is about half that away. The tail gives the reverberation time (RT60), how long the room takes to go 60 dB quieter.

Both devices need the same code. Use the louder speaker as the beacon.

How far apart two devices are, by sound alone: one plays a code, the other listens. Start the two side by side, start listening, and hold still for a few seconds while the listener measures how far the two clocks drift apart; then move. The code only repeats every 234 m of path, so every reading is absolute; each extra 0.1 ms of delay is 3.4 cm. The listener follows the earliest strong arrival (the direct sound, even when a reflection is louder) and skips readings that jump further than anyone could move.