(Web Audio API) Oscillator node error: cannot call start more than once

A better way would be to start the oscillatorNode once and connect/disconnect the oscillatorNode from the graph when needed, ie :

var ctx = new AudioContext();
var osc = ctx.createOscillator();   
osc.frequency.value = 8000;    
osc.start();    
$(document).ready(function() {
    $("#start").click(function() {
         osc.connect(ctx.destination);
    });
    $("#stop").click(function() {
         osc.disconnect(ctx.destination);
    });
});

This how muting in done in muting the thermin (mozilla web audio api documentation)