I got this response from the polyfill developer:
if (typeof window.waapisimContexts != 'undefined'){
loadSound("audio/bird.wav");
} else {
loadSound("audio/bird.mp3");
}
The polyfill creates waapisimContexts only if the browser requires it to play a .wav, so this approach looks for the waapisimContexts and loads the .wav if it is defined. Otherwise, it loads the .mp3.