In Firefox you can work around this. Click on the padlock next to the URL, then click the Right Arrow, and More Information. Head to the Permissions tab, scroll down and find Autoplay, and switch it to "Allow Audio and Video." Refresh, and you should have sound.
For whatever reason Firefox blocks it, tells you in the console that it blocked it, but fails to present the extra little UI in the site info dropdown that would usually expose the user override. Might be a bug with how this particular game sets up the web audio context or something.
Firefox prevents audio from playing with no initial user interaction like a play button. Can see the warning if you pop open the dev console. Don't know of workaround besides sites not creating an AudioContext on page load.
Edit: Keyboard input does not work :/ it was a good effort.
Found a workaround throw it in an iFrame and have the frame load with a user interaction. Here is a jsFiddle link, just click "Run" after the page loads.
You can explicitly allow it in Site Settings, and then reload the page. For me, I click the little button (looks like sliders) to the left of the URL, Site Settings, and then scroll down to change Site Audio to "Allow".
But still awesome seeing the game pop up instantly with no loading, prompts or whatever. Just like turning on an actual n64.