A developer publishing as m-c-tech found that his multipoint Bluetooth headphones stopped switching audio back to his phone whenever an AliExpress tab was open in Firefox or Chrome. Closing the tab fixed it at once. There was no video on the page and nothing audible coming out of it; muting the tab, the browser and Windows changed nothing. The trouble did not begin on load, either: it appeared after the home page had been sitting idle for several seconds.
The usual suspects showed nothing. No audio or video elements, no calls to play, no active media session. So he wrapped the AudioContext constructor and the method that connects audio nodes, loaded the page again, and watched two hidden contexts appear while it sat idle. Both were running, and both were connected to the system's audio destination. The stack traces named two obfuscated scripts belonging to Alibaba's anti-abuse tooling.
The graph both scripts build is the same: a sawtooth oscillator into an analyser, then a processor, then a gain node set to zero, then the speakers. Nobody hears it. The browser still has to process the graph in real time, and the analyser reads back how this machine rendered a known signal. That is why the mute button was beside the point and no speaker icon ever appeared: mute acts on playback objects, and the page had created none. Firefox has the underlying behaviour on file as bug 1863193.
The audio reading is one measurement of roughly twenty the same bundles take. Canvas, WebGL, screen, hardware, plugins, WebRTC, timing, pointer and scroll events, device motion: serialised, encrypted and sent on to Alibaba's telemetry. By itself the tone is worth very little, and Tom Ritter of Mozilla has measured how little; combined with the rest it is a fingerprint that survives clearing cookies. Blocking the two scripts in uBlock Origin stopped the contexts appearing, at the risk of more CAPTCHAs at login.
The issue is MUTE POINT, an original cartridge for the Nintendo Entertainment System. Six identical display cases stand along a shop counter. Stand at one long enough and your own music stops while a tone that is not yours plays in your headphones, and you have forty seconds to name the case that did it. B mutes the shop and does nothing to the tone. Play it in the browser, or run the ROM on real hardware.