DeaDBeeF stable build 1.8.1 support building with GTKNativeFileChooser which is utilized to support flatpak portals. Other than that the xdg-music (~/Music) folder is made available by default so it is not fully sandboxed.
The master build manifest (music.deadbeef.player-master.json) builds DeaDBeeF with support for GtkFileChooserNative which gives portal access to single files.
Portals does now support opening folders, but there is still no logic to automatically request both files in a cue/mp3 set (or similar). Opening a folder containing such a set does work but is counterintuitive. xdg-desktop-portals project is looking into this. Follow this issue for more updates.
This flatpak is not using --socket=fallback-x11 in order for hotkeys plugin to work properly. Thus X11 is always available even on Wayland. It needs to be patched to check if X11 is available at runtime. It uses raw Xlib calls to implement global hotkeys which isn't working on Wayland anyways.
You can allow full filesystem and network share access by using overrides like so:
$ flatpak --user override --filesystem=host --filesystem=xdg-run/gvfs music.deadbeef.player
This flatpak uses my improved pulseaudio output plugin and does not include the one included with DeaDBeeF by default, nor does it include ALSA.