Audio preloading

I’m using wp_audio_shortcode() to display an audio file in my page. For example:

$attr = array(
    'src'      => 'http://example.com/my-audio-file.wav',
    'preload' => 'auto'
    );
echo wp_audio_shortcode( $attr );

I’ve used the preload option to load the audio entirely when the page loads.

I can confirm that the metadata is loaded fully (duration, mime type etc).

My question

Does the entire audio file always download fully or just a portion of it? I’m asking because on big .wav files in particular, it’s noticeable that the time-loaded indicator is never fully at 100%. Yet on small .mp3 files, the time-loaded indicator always reaches 100%.

Please note I’m not referring to ‘time-played’.

Ref: http://codex.wordpress.org/Function_Reference/wp_audio_shortcode