The package requires proprietary software and instrument data to compile the music, which is therefore proprietary.

Source: <https://github.com/singularity/singularity/tree/master/singularity/data/music-src> references:
- Psycle, free software which only works on Losedows, or a compatability layer like WINE.
- Drumatic VE VST, proprietary software.
- Renoise, proprietary software, although apparently a free, deprecated PHP script also works.
- "commercial instruments", proprietary instrument data. Note also the conflation of freedom and price.

However, the project does provide precompiled OGGs and FLACs, the former of which are packaged at the moment.

I would propose blacklisting the package by removing the music for the moment, although on a technical note, the package should be split into singularity, singularity-music, singularity-music-lossless-original, and singularity-music-lossless-extended, so as to allow the user to choose which one to install.
The only reason I suggest making technical improvements to a partially proprietary package is so as to separate the free from the nonfree, which would allow Parabola to easily blacklist the singularity-music* packages instead having to maintain a libre, cleaned-up package, which would free up developer time for other tasks.
This particular issue should probably be forwarded upstream to Arch.