Ah, I didn't know libvgm is this far these days! I hope they do, I think it stings a bit this site that is entirely about files in the VGM format offers MP3 files for playback. And it will save a lot of space.
I hope one day webbrowsers on mobile phones will provide good background audio playback, with the screen off. If that'll work you won't even need an app, just a site. Not sure if there's a business model in that for Google and such. Maybe it's the case already... haven't looked into it for a while.
If you want to do it yourself, look at the new webaudio spec as well. The scriptprocessor used to be deprecated with no alternative available, pretty odd, but now there is with audioworklet.
Nice, this other player... lots of efforts on the web... still missing the ultimate MSX music archive which includes a playback option as well though. I'm still hoping this someday will be included overhere though. 