I've begun compiling a playlist of "in-universe" music for my future campaigns to get a sense of the "soundscape" of the game world and the kinds of tunes that might earworm an NPC, like the guard humming Podmoskovnye Vechera.
https://music.youtube.com/playlist?list ... ture=share
Currently, it's a mix of pre-2000 American, Polish and Russian pop songs. It's not strictly diegetic -- some of the tracks are post-2000, remasters, re-recordings, or covers. It's already 6+ hours, but I'm looking to add more folk (like Podmoskovnye Vechera), country, jazz, R&B and hip hop -- not to mention all the music that would be playing in Sweden!