Ultibo has now a support for VC4 - OpenGL, etc stuff.
This thing
https://github.com/pik33/ultibo_retro_gui is an alpha stage of window system I wrote before VC4 availability and it will be now rewritten using new possibilities as soon as I can learn how to use them.
Although this is alpha stage project, it contains working audio player with Winamp Classic skins, which plays some audio formats - mp2, mp3, wav, C64 SID ans Amiga MOD files. Having full control over the hardware I managed to output high quality audio through RPi's jack using 20x oversampling and noise shaping. No compilation required, the repository contains the binary file.