[eirki]

After spending most of the evening with this I'm guessing Kodi doesn't support adding strm files to music library at all. I might be wrong but

2 Adding STRM files to the library

You can associate meta-data, such as cover art and summaries, to a .strm file just like normal entries in a library by using an NFO file, just as you would for a normal video file.

There's no standard for adding music metadata to specific strm file other than nfo and nfos are used only for artist and album information.

Not sure if this is useful information but you can add metadata to specific songs by using cue sheets. They are text files (actually .cue files) that are in functionality very similar to NFOs. Just make one cue file for each song, contaning the relevant metadata.