Do the stock music files work? Ie Gravity (internal percussion) and Meltdown? If they do, check the files you sent. MPD should accept a variety of file types and bit rates, but check the documentation.
https://www.musicpd.org/
I haven’t modified anything with the MPD, so this is interesting. Perhaps try putting the volume up. I may have had my image set to zero volume upon making my images as I work on things while people are asleep.
Could you post up one of your music files for me to test? The only other thing I could think is that something went wrong in the expansion process. Try things out on a completely stock image.
As for the doom files etc, you will need to have the WAD files. Put them in the games/chocodm folder. Note: problem occur when you use the steam versions. For any other installation or file sourcing queries, check this link out: https://www.chocolate-doom.org/wiki/index.php/FAQ
Also have a look at the link I posted above re: chocodoom etc. Running Doom and Heretic
note, this is extremely out of date, and all of the directory structure references are incorrect. I’ve fixed them in my image.