Everything else, including movie spinoffs starring the same actors who appeared in the original series, is ITS OWN canon, as a spinoff, adaptation, or what have you.
Ok, so let me see if I have this straight. The Star Trek films are not canon? Ok. Spock appears on TNG and references the events of Star Trek VI. I think that makes STVI a part of TNG canon.. Zefram Cochrane is shown in STTNG First Contact. The same actor appears as Zefram in Enterprise. Star Trek novels reference ALL Trek series, sometimes mixing and matching. With Trek at least, the movies and subseuqent TV series are all part of an intricate continuity, even if it's consistent at times.
Now with DS, I consider the audio dramas to be canon. They use the original actors and make an effort not to contradict the original show. That's my opinion. You won't change it. Feel free to waste your time if you wish.
In the end, it's a moot question any way. Who decides what's canon? Do we set up a council and vote? Do we let the Curtis family decide? Do they care? Would we trust their answer? In the end, canon is what we individually decide it is.
With Star Trek, we have Paramount deciding. They have decreed that ALL aired Trek and the movies are canon EXCEPT the animated show. Why it's not, I don't know. We found out there that the Klingons had the cloaking device. We saw the holodeck, or a reasonable facsimile there first. They showed us more of Vulcqan and "Yesteryear" was directly alluded to in "Unification I" (the episode in which Sarek dies and Picard searches for Spock". We got stories that were at least as good as the average Trek episode and NONE sunk to the depths of "Mark of Gideon" and "Spock's Brain". That of course is very debatable I hate those episodes. Someone else may love them and hate the animated show. But with Star Trek, we have a body dictating what the comic and book series can use and do and what they can't. The proof that the books and comics are not canon is the fact that they generally wildly contradict each other even though each project is generally consistent with the various series.
Don't try to tell a Whovian that the two Doctor Who series aren't both canonical! Images of all the past Doctors have been shown on the new show.
The reason I consider the CDs to be canon is that they use the original cast. I don't discriminate based on the fact that they are audio. If they were live action TV, I believe they would have wider acceptance. I would consider counting the new comic as well if it stays consistent with what Big Finish is doing.
I never watch '91 DS without reading the Innovation comics. I consider that to be part of the all to short continuity.