I am glad I found you all. I have been doing some tinkering with BBB and raspberry Pi for audio and I had found Volumio but not your project until a post somewhere referenced you.
I understand a fair amount about how the parts of this are all working, but I think we would all benefit from more documentation of what is going on.
As far as I can tell, we have a stripped down distro, essentially a server distro for ARM6 or ARM7. On top of that a LAMP like stack is installed + SAMBA (and similar) + MPD.
On top of all that we have the PHP web application, that functions both as a MPD client and also lets you configure things like SAMBA.
I am particularly interested in what has been changed in the OS. Are you simply starting with the normal Arch base and adding the minimum to get your web interface going or have you really forked Arch? Has the kernel been recompiled, if so what changes were important?
I don't know what your plans are but I did notice the RuneOS git was empty.