As I've been involved in WWII living history for over 10 years, music is very important for me when playing SH3 :P This is relevant to all versions of Silent Hunter I suppose but wasn't too sure where to post, so apologies to the moderators if this needs moving.
I know there's plenty of mods out there and I'm guessing there has been some other threads covering this, but I recently stumbled across the U-Boat Society living history group in the UK, and they have quite a nice list of music as MP3 on their site
http://www.theuboatsociety.com/page7.html (just noticed the links don't actually work... the link is to the chap's hard drive... mighbt be a bit difficult to play them from there :P But I suppose a search for the file names will bring up a version of the song at least).
Also, whilst not all relevant, might I also recommend Joeri's Radio Files -
http://www.joeri.net/radiofiles/ - there is a small fee but there's radio broadcasts from several nations, military broadcasts, music collections, black propaganda, 10 hours of "swing verboten" - 217 songs that made it into Germany in the years 1923 to 1944, radio hits for every year of the war (I am particularly fussy about playing a 1944 song in 1943, otherwise what's the point of 100% realism :P), and there's also Großdeutscher Rundfunk broadcasts - although I know there's a mod for that out there for SH5 at least. Anyhoo, check them out, they have tonnes of stuff