After a few months of puttering around, I'm ready to call version 1.6 stable. I've also added the project to PyPi, so you should be able to install it with the command:
pip3 install somafm
A lot of new things have been added in version 1.6, but the biggest one is probably support for other media players (mpg123/mpv) in addition to MPlayer. The script will still use MPlayer by default, but if it's not installed and one of the others is, there's now a fallback. There are some drawbacks though, as mpg123 doesn't support high-quality AAC streams and mpv is slow and overall a bit janky. But at least the option is there.
You can now turn off track sync when streaming to Chromecast devices, which is good if you want to play media "headless". Desktop notifications won't pop up for Station IDs (oh, and I fixed broken desktop notifications), and there's even some experimental support for custom notification commands which could potentially be used for things like showing track data on an external display (that's what I'm planning on, anyway).
Give it a shot and let me know what I broke.