What this is for
Plays a Web Audio tone, a tiny WAV through an audio element, and a spoken word. One path can work while another is blocked.
Computer audio fails in layers, and the layers have to be peeled in order. Hardware mute, the wrong output device, a muted browser tab, a site permission, then a dead jack or a driver. A tone from this site only proves the path from the tab to the device the operating system has selected. It does not test a speaker that is plugged into a different jack.
Laptops often say stereo and then put both channels through grilles a few centimetres apart. If left and right are hard to tell apart while you sit normally, lean in and listen at each grille. A tone that comes from both grilles no matter which button you press means the machine has summed to mono, or you are hearing structure-borne sound rather than two drivers.
How to use the result
Windows keeps the choice under Settings, System, Sound, Output. The speaker icon in the taskbar is the quick mute and the quick device switch. Windows also has its own test button on that settings page. Use it. If Windows is silent and this page is silent, the browser is not the interesting part. If Windows plays and this page does not, look for a muted tab, marked by a speaker with a slash.
On a Mac the same choice is System Settings, Sound, Output. Option-click the menu-bar volume icon when you do not want to open Settings. Volume keys should move the device you selected. A USB interface and the MacBook speakers are different outputs, and a browser will not jump between them because a page asked nicely.
Monitors are outputs only when the cable carries audio. HDMI and DisplayPort can. Many USB-C adapters do not. The clue is simple: if the monitor never appears in the output list, the path is not there, and no test tone will invent it. If it appears and stays quiet, turn the monitor’s own volume up. Those knobs often start at zero while the picture is fine.
What it cannot claim
USB audio devices add a sample-rate trap. A interface stuck at 48 kHz and a system that opened the device at 44.1 kHz can look connected and play nothing, or play at the wrong speed. Select the device explicitly, then play a 440 Hz tone. You know what 440 Hz sounds like if you have just heard it on the laptop speakers. A large pitch error is a clock or a rate mismatch, not a “warm” converter.
The device list on this site is the browser’s list, which is not always the operating system’s list. Names stay blank until a permission has been granted at least once. Showing the list does not switch the default. After you change the default in the system panel, play the tone again. The useful result is boring: you hear it from the thing you just selected.
External speakers on a 3.5 mm jack still depend on that jack being the selected device, and on the plug being fully seated. A half-inserted plug is a classic one-side failure. If the headphone page shows one side dead and the laptop speakers play both, the fault moved with the plug. Follow the hardware, not the most recent website you opened.
Practical notes
When you change devices, play one 440 Hz tone and listen for where it comes from, not for whether it sounds impressive. A USB headset, a monitor, and the laptop speakers can all be “connected” while only one of them is selected. The browser test and the system test should agree. If they disagree, the tab is muted or a different device became the default between the two clicks. Set the default, play once, and stop.