@dang Maybe you could add a [Loud sound] tag on the title. I clicked on it without knowing and blasted a huge noise from external speakers at midnight.

Wife and neighbors are not happy.

Why do (some?) browsers still let JS autoplay audio but not plain <audio> elements?

Didn't do it for me. To test my headphones and ears, I tested a 50hz waveform on Youtube and could hear it.

Didn't work for me at first, but I moved on the website pages and it did.

According to http://hummingbirdclock.info/static/js/hum.js the base frequency is 200 Hz, but has a gain of 4, which seems to push it more into the higher harmonic range as the waveform is clipped:

    // hum.js
    
    // toggle html audio on/off
    // with cookie to maintain state
    // and use html5 audio synthesis
    // ios html5 audio must be triggered
    // call init_hum() after DOM loaded
    
    // 0. create audio context
    // 1. create oscillator, define
    // 2. connect oscillator to gain
    // 3. create gain
    // 4. connect gain to output
    // 5. start oscillator (on click)
    
    var audio;
    var audio_context;
    var control;
    var vco, vca;
    var hum_delta = 1;
    var hum_base = 200;
    var hum_min = 100;
    var hum_max = 400;
    
    /* init */
    
    function init_hum() {
        audio = get_cookie("audio");
        console.log("audio = " + audio);
        audio_context = get_audio_context();
        console.log("audio_context = " + audio_context);
        control = get_control();
        if (audio_context != "false") {
            if (audio != "off") {
                set_hum();
                hum_on();
            }
        } else {
            set_cookie("audio", "off");
        }
    }
    
    function get_audio_context() {
        var which_audio_context = window.AudioContext || // default
                                  window.webkitAudioContext || // safari
                                  false;
        this_audio_context = new which_audio_context;
        return this_audio_context;
    }
    
    function get_control() {
        var this_control = document.getElementById("control");
        this_control.addEventListener("click", hum_on_off);
        return this_control;
    }
    
    function set_hum() {
        vco = audio_context.createOscillator();
        vco.type = 'sine';
        vco.frequency.value = hum_base;
        vca = audio_context.createGain();
        vca.gain.value = 4.0;
        vco.connect(vca);
        vca.connect(audio_context.destination);            
    }
    
    /* on off */
    
    function hum_on() {
        set_hum();
        vco.start(0); 
        control.innerHTML="×";
        set_cookie("audio", "on");
        audio = get_cookie("audio");
        console.log("audio = " + audio);
    }
    
    function hum_off() {
        vco.stop(0); 
        control.innerHTML="+";
        set_cookie("audio", "off");
        audio = get_cookie("audio");
        console.log("audio = " + audio);
        cleanup();
    }
    
    function hum_on_off() {
        if (audio == "off")
            hum_on();
        else
            hum_off();
    }
    
    function cleanup() {
        vco.disconnect(0); 
    }
    
    /* cookies */
    
    function set_cookie(cname, cvalue) {
        document.cookie = cname + "=" + cvalue;
    }
    
    function get_cookie(cname) {
        var name = cname + "=";
        var ca = document.cookie.split(';');
        for(var i = 0; i <ca.length; i++) {
            var c = ca[i];
            while (c.charAt(0)==' ')
                c = c.substring(1);
            if (c.indexOf(name) == 0)
                return c.substring(name.length,c.length);
        }
        return "";
    }
    
    function check_cookie(cname) {
        if (getCookie(cname) != "")
            return true;
        else
            return false;
    }

The audio spectrum analyzer on my phone (Spectroid) records it most strongly around 1000 Hz.

At 50Hz it would be inaudible on most computers.

https://szynalski.com/tone#50,v1

True, but cheating a bit with a triangle wave would solve it thanks to the harmonics.