Part of this at this link:
https://sites.google.com/site/middleschoolandroid/home/app-inventor-tutorials/music--player--working-with-screens This shows how to play music files with AIClassic. it is very similar for AI2.
The random part: instead of selecting with individual names, use numerical tags on the TinyDB and call the songs with the random number function in the Math blocks.
You will see random integer from 1 to 100 change the "100" to reflect the number of mo3s you have in the tinyDB.. Whatever random integer from 1 to 10 (if you have 10 songs ... I am not certain you can use that many but perhaps you can) is the tag name for the mp3 tag number .you select from the tinyDB ..... similar to how they do it in the above example.
This will require a bit of work. If you run in to difficulty, show your blocks image hfere and someone will help.
Regards,
Steve