If you have purchased the Premium Edition of DOOM: The Dark Ages, the Digital Artbook and Original Soundtrack are installed through a separate Steam listing. Open your library and search DOOM, include Tools in the filter. A listing for DOOM: The Dark Ages Digital Artbook and Original Soundtrack can be found. Install the application and launch it to view the content.

The Soundtrack can be found in the File Explorer after installing the tool. The .wav files can be found at: \Steam\steamapps\common\DOOM The Dark Ages - Digital Artbook\DOOM_TheDarkAges_Artbook_Data\StreamingAssets\Generated\Soundtrack