Could it be that this is a known bug with ffmpeg.wasm? Other folks are also getting 0-byte empty files when attempting to convert and mp3 to ogg when using the libopus codec. You could try their suggestion, which is to use libvorbis intead. To do so, you could change the Zustand store from:

{
  acodec: "opus",
  outputFormat: "ogg",
  bitrate: "128k",
  // ... other settings
}

to:

{
  acodec: "vorbis",
  outputFormat: "ogg",
  bitrate: "128k",
  // ... other settings
}