Question about Bose SoundLink replacement audio cable

But it looks like you can't adjust the volume on the QC25 with it ...

Yes, it should be such a cable. I had actually wanted to order it as a normal AUX cable, as it looked like one to me. But then it turned out that it was thinner on one side than the other. Was able to return it without any problems. The description says it fits the QC35 etc. Should most likely fit your QC25 as well.