[miscz]

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.