According to a thread on steam, which the developer answered, the soundtrack is included in the base installation itself and the files are available if you browse the game's local files:
MACAXEIRA Jun 18, 2013 @ 1:58pm
where can i find the game soundtrack?
prunk [developer] Jun 18, 2013 @ 2:04pm
It's on the Knights instalation folder. You can find it by right-clicking on the game on your Steam Library>Properties>Local Files tab>Browse Local Files.
The thread is specific about the Mac installation, but there's no indication that the Windows installation should be any different.
After a bit more investigation, I've figured out the following.
The previous information posted by the developer was correct in 2013 (and why I have the digital files of the music 🤣 ). When the game was first made available, the soundtrack files were simple standalone files within the game installation and you could browse the local directory to find them.
However, since then the game has had numerous updates through the publisher, Paradox Interactive. At some point around 2014, the soundtrack files stopped being available. It turns out that the way the game was originally packaged for sale, the soundtrack was considered a "DLC" or expansion. The +1 Edition available on Steam does not have the soundtrack. The "Deluxe Edition" has an expansion/DLC, but not the soundtrack. The archive.org listing of steamapps in the comments shows the expansion was once available, but the publisher decided to remove it for whatever reason.
The developer (not the publisher! two separate entities here) confirmed this in a thread and said that the way to officially get the soundtrack was through the Bandcamp page.
gty [developer] Apr 1, 2014 @ 3:00pm
When I said you can't it's because in the current build/version of the game that can't be done. Sorry for the kind of misleading info.
I asked my friends here and I honestly don't know why the soundtrack isn't listed on humblebundle. Paradox might had a specific deal with them maybe? Carl (Paradox producer) once posted here regarding the DLC that they are currently checking the possibility of this upgrade button.
You may directly buy the soundtrack here: http://beholdstudios.bandcamp.com/album/knights-of-pen-paper
(Recall that the PC version was initially offered through and gained popularity from a Humble Bundle offering.)
Anyway, there's a massive Q&A thread that you can search and the topic does drift back to the soundtrack a few times, but at the end of the day the answer seems to be that the publisher didn't make it available via Steam.