One way (not the most straightforward) to get Spotify sound
over to icecast on FreeBSD:


In one terminal of the FreeBSD machine:

ssh -N -T -L 8000:localhost:8000 youricecastserver.net


In another terminal on the FreeBSD machine:

/home/username/.cargo/bin/librespot -j -b 320 -R 90 -n librespot -B pipe | /usr/local/bin/opusenc --quiet --raw --raw-bits 16 --raw-chan 2 --raw-rate 44100 --bitrate 256 - - | \
/home/username/bin/oggfwd localhost 8000 source yourpassword test.ogg


Then tell

spotify-qt

to use librespot as a device and play your songs or playlist.

(Note that it seems one has to use an updated version of librespot.
librespot needs to get updated more often than quarterly branch.)

librespot won't work without a Spotify subscription.

This sort of works for me right now.

Another way which works for me sometimes is the application

butt

combined with

Spotify under linux emulation.

It's usually a hassle to do this either way.