Successfully built via nix-shell -p nixpkgs-review --run "nixpkgs-review pr 96895".
It also runs but doesn't seem to work. I only get a bunch of pulseaudio errors:

Module module-null-sink not loaded.
Module module-remap-source not loaded.

Those are normal, you'll need to put "Toggle Lyrebird" into the on position, then play around with the presets / pitch shift value while having something listening to the newly created "Lyrebird Virtual Input" device.

Tested with Discord, device gets picked up and successfully applies the selected presets.

A warning for all possible reviewers: This PR was branched of while the fontconfig font cache issue was present in master. If the system behaves weird after nix-review, run fc-cache -r.

Rebased on current master, I hope that helps with the fontconfig cache issue.

getting these errors when trying to use it:

sox FAIL formats: can't open output file `Lyrebird-Output': can not open audio device: No such entity
No PulseAudio daemon running, or not running as session daemon.
Traceback (most recent call last):
  File "/nix/store/j475am9xsjzyvxjdgab6xmljj1l1dk2b-lyrebird-1.1.0/share/lyrebird/app/mainwindow.py", line 162, in toggle_activated
    null_sink = subprocess.check_call(
  File "/nix/store/bs03sg8b0gq2zr4v252hh9psp780qj5q-python3-3.8.5/lib/python3.8/subprocess.py", line 364, in check_call
    raise CalledProcessError(retcode, cmd)
subprocess.CalledProcessError: Command '['pacmd', 'load-module', 'module-null-sink', 'sink_name=Lyrebird-Output']' returned non-zero exit status 1.

wat do I need to do? otherwise looks good

Added pulseaudio to PATH so it can find pacmd, blind guess that's what's missing? If that didn't fix it then I'd suspect the application having a problem with your audio setup: are you running Pulse & does pacmd load-module module-null-sink sink_name=Lyrebird-Output complete without errors if you run it?

hmm, it does indeed not work. I guess my setup is a little bit unconventional with pulseaudio running systemWide

That does appear to be the problem, as lyrebird tries to load PulseAudio modules via pacmd load-module. Having a system-wide PulseAudio server seems to not be compatible with that approach by default, assuming none of the basic design behind this 13 year old report has been changed.

Normally the automatic module loading bit will fail with system PAs
because module loading is not allowed and thus the module-cli-protocol-
unix cannot be loaded.

Either way, this is by design and I'm not sure it's something we should
change, especially as system mode is not really recommended or really
heavily tested upstream.