On NixOS unstable, at some point my Bluetooth headphones (WH-1000XM4) stopped producing sound after boot. I tried to track down the cause together with ChatGPT, but couldn’t fully fix it. In the end, I worked around it by restarting the audio services after Niri starts:

spawn-sh-at-startup "systemctl --user restart wireplumber pipewire pipewire-pulse"