When using a MediaElementAudioSource node and connecting to an AudioNode, there is no change in audio, as if the AudioNode is being circumvented.
As per the spec for createMediaElementSource:
As a consequence of calling this method, audio playback from the HTMLMediaElement will be re-routed into the processing graph of the AudioContext.
This has a MediaElementAudioSource -> Gain (with gain @ 100) -> destination
In Chrome, you can hear the distortion -- in Safari, there is no change.
This works fine when using a different source, such as an oscillator or AudioBuffer.