My playlist includes a mix of German military music from that era and before, plus popular jazz/vocal recordings that I know or assume were likely to have been available at the time. Lots of Comedian Harmonists, a popular German vocal group from the 30s, and jazz and big band stuff. Some German/European but also a lot of American stuff provided it's something that predates the US entering the war, at which point I assume it's highly unlikely that new US releases would end up in the private collections of German u-boat commanders.
Also a lot of classical music since in theory some recordings of those works would've been readily available as well, even though most of the recordings I use were actually made decades later.
Basically I try to achieve some level of realism that's acceptable to me based on my own imperfect, arbitrary system of deciding what music might have been available. But I don't sweat it if a "record" that wasn't released until 1940 shows up on my boat in 1939.
