Step 1: Navigate to the Bonus Content's install folder (this is most easily done by right-clicking it in your Steam library, choosing Properties and view local files.

Step 2: Follow the file path windata > 201 > stream

Step 3: Copy every music track you want. They should run in any music player that accepts .m4a format music files.

If you pre-ordered: You'll have access to the orchestral tracks. From the windata folder choose DLC and the first folder "2431340" contains the pre-order tracks as noted by the identifier text file in there. Copy them to wherever you want. The files haven't had the extensions assigned so right-click, properties and add .m4a to the file name and click Ok. This'll assign it to that file format. I tried assigning .mp3 but it caused brief freezing in my media player. If you know a better extension that works, let me know in the comments.

The tracks for those that want specifics:
File beginning:
121 - Snake Eater (MGS3)
345 - The Best Is Yet To Come (MGS1)
566 - Can't Say Goodbye to Yesterday (MGS2)

If .m4a files aren't your preferred music file format, you can use music converter sites/programs or audio editing software like Audacity (which is free) to convert the tracks to whatever format you prefer. (Thank to people who commented this as I hadn't thought of this.)

The graphic novels
- The other DLC folders contain the graphic novel folders.
- The first 2 are the MGS1 novel and the second 2 the MGS2 one, with the first folder in each pair being the Japanese version.
- Each folder contains 2 files. The bigger one is the novel itself and the smaller a short introduction trailer that's unimportant to the novel itself.
- Copy each file and add the file extension .m4a to each copy, like with the music files, to make them watchable as video.
The resolution isn't great so if anyone knows a file extension that'll improve that, let me know in the comments.