The long and short of it is that we wanted to keep things as tidy as possible, but this stack doesn't get nearly as many questions as we had hoped. As such, we don't really adhere to the suggestions as strictly as we wanted to. Re-tagging is fine, as long as the re-tags make sense and aren't "forced" re-tags.
I'd say that they are not synonyms, but closely germane.
A question like
What does define Pink Floyd's sound as 'experimental'?
would be tagged with sound.
Whereas a question like:
Why is the sound quality of a track reduced when it is compressed into a .mp3?
could be tagged with sound, but sound-quality fits better here
I agree with Bebs that unknown-performer and unknown-composer are being misused. However, I think Brahadeesh is correct that the unknown-composer questions should be re-tagged as identify-this-composer.
I think Bebs may be referencing his answer largely to the contemporary world of music, which is performer-focused, where many people sing their own songs,...
Thanks for bringing this up! I'm surprised that this discussion hasn't already been had.
I'll start by noting that the situation today is far better than it was a year ago: nearly half of the questions tagged [song-title] were also tagged [identify-this-song] until the conflicted tagging was cleaned up; see my earlier Meta question:
"Please don't ...
It turned out that this question had two separate issues under discussion: (1) how to use the (unknown-composer) tag correctly, and (2) whether we need an (identify-this-composer) tag.
Based on the previous answers, most of the example questions cited in the OP have been retagged, based on the following understanding (taken from @Bebs' answer):
There is no need of "identify-this-composer" tag since we already have identify-this-artist. By artist, it could mean singer, musician, dancer, back-up extra, composer, etc. We shouldn't have too much of this tags or it would be too granular.
Using unknown-composer to identify a song is most of the time mis-tagging since like unknown-performer, the tag ...