I realize I am replying a bit late, but after some hunting (due to having the same issue myself), I have found a couple users saying that the OST does not install anywhere anymore for users. I found a contact email for the dev team and sent them a message, and am hoping they will respond, assuming the email address is still active.

Just an update: The dev wasn't sure why I (and presumably some others) are having issues with the soundtrack not downloading anywhere, but I did get a very prompt reply when I emailed and they are now aware of the situation. :) (I also was able to get a copy of the OST via emailing them at the link on their website, but ofc that might not be a resolution that will work for others with this issue.)

Ah, sorry, figured this out. It was an older DLC-style soundtrack and Steam "automatically converted" it in a way that completely broke it. It's fixed now.

Thank you for the update. I could find the soundtrack on steam now!
And congrats on Caves of Qud 1.0!