This sample shows how to implement a clip-indicating volume meter in Web Audio, using a ScriptProcessor. It's necessary to use a ScriptProcessor in order to not miss any clipping samples - otherwise you could implement this using a RealtimeAnalyser to only grab samples when necessary.

max

average

Check out the source on Github.

The usage is quite easy:

var meter = createAudioMeter(audioContext,clipLevel,averaging,clipLag);

audioContext: the AudioContext you're using.
clipLevel: the level (0 to 1) that you would consider "clipping".  Defaults to 0.98.
averaging: how "smoothed" you would like the meter to be over time.  Should be between 0 and less than 1.  Defaults to 0.95.
clipLag: how long you would like the "clipping" indicator to show after clipping has occured, in milliseconds.  Defaults to 750ms.

meter.checkClipping();

returns true if the node has clipped in the last clipLag milliseconds.

meter.shutdown();

used to destroy the node (it's important to disconnect and remove the event handler for any ScriptProcessor).