Hi John,
you need to download a static version of FFMPEG,
put it in the folder “RoonServer/RoonOnNAS/bin” on your NAS and
restart the RoonServer package.
Link to FFMPEG (the same version as recommended for use with ROCK):
https://johnvansickle.com/ffmpeg/releases/ffmpeg-release-amd64-static.tar.xz
Unpack (maybe unpacking twice is needed) and copy only the ffmpeg-file into the above mentioned folder.
Actually you will find a .txt file within the folder explaining the same thing.
Hope this helps!
Regards, Roland