Are you trying to use your custom button to trigger the mejs button? Can you provide the file for me/us to tinker with?
You should be able to look up the Dom tree and find something with audio in it the you can add to what you have. You might need an Nth child selector. I sense a bit of interest, check out jQuery Selectors. In another thread you asked about jQuery vs. Vanilla JS when used in Lectora (I added the last bit), well, this is one of the main reasons why. Look at all those awesome selectors that you can use to pin down your target. Doing those with vanilla JS is possible, some require a lot of code but.. why reinvent the wheel?