Sorry...as ever it was a stupid mistake that made it not work. Audio for HTML5 mobile devices (both Android and iOS at least, I don't own a Windows phone) does work in GM:S.
So annoying that I've spent about 2 weeks trying to figure out audio when the fix was...well not staring me in the face, but was really wasn't as tricky as I thought it was.
So for anyone else struggling out there who wants to do this, you don't need a .js extension or anything like that to play HTML5 audio in GM:S.
- Add a sound resource to your project, make sure it's a format that can be played by the mobile device (i.e. .mp3 for iOS & android) as stated
http://gmc.yoyogames.com/index.php?showtopic=606885&p=4487745
- For your sound resource, at least for me, under Properties, I could only get sound to play under the "Compressed not streamed" setting with correct sample rate.
- Use the new Audio Engine not the legacy one and write that if else loop like the documentation tells you to capture any legacy devices so it they can have audio - don't discriminate
- Create a Left Mouse Release event for the area of screen you have assigned for this and run the code audio_play_sound(snd_bkg, 10, false); for example
Audio will work.
Phew!!! Can move on with my life now!