adding sound emulation in browser

sim/libmbit.ts

@@ -332,13 +332,69 @@ namespace ks.rt.micro_bit {
     export function servoSetPulse(pin: Pin, micros:number) {        
     }
         
+    module AudioContextManager {
+        var _context : any; // AudioContext
+        var _vco : any; //OscillatorNode;
+        var _vca: any; // GainNode;
+        
+        function context() : any {
+            if (!_context) _context = freshContext();
+            return _context;
+        }
+        
+        function freshContext() : any {
+            (<any>window).AudioContext = (<any>window).AudioContext || (<any>window).webkitAudioContext;
+            if ((<any>window).AudioContext) {
+                try {
+                    // this call my crash.
+                    // SyntaxError: audio resources unavailable for AudioContext construction
+                    return  new (<any>window).AudioContext();
+                } catch(e) {}
+             }
+            return undefined;
+        }
+        
+        export function stop() {
+            if (_vca) _vca.gain.value = 0;
+        }
+        
+        export function tone(frequency: number, gain: number) { 
+            if (frequency <= 0) return;            
+            var ctx = context();
+            if (!ctx) return;
+            
+            gain = Math.max(0, Math.min(1, gain));            
+            if (!_vco) {
+                try {
+                    _vco = ctx.createOscillator();
+                    _vca = ctx.createGain();
+                    _vco.connect(_vca);
+                    _vca.connect(ctx.destination);
+                    _vca.gain.value = gain;
+                    _vco.start(0);
+                } catch(e) {
+                    _vco = undefined;
+                    _vca = undefined;
+                    return;
+                }
+            }
+            
+            _vco.frequency.value = frequency;
+            _vca.gain.value = gain;
+        }
+    }
+    
     export function enablePitch(pin: Pin) {
         pin.mode = PinMode.Analog | PinMode.Output;
     }
     
     export function pitch(frequency: number, ms: number) {
         let cb = getResume();
-        setTimeout(() => { cb() }, ms);
+        AudioContextManager.tone(frequency, 1);
+        setTimeout(() => {
+            if (ms > 0) AudioContextManager.stop();            
+            cb() 
+        }, ms);
     }