Hello VonReichel,
Being a musician, this question intrigued me and I've been researching it. It's really easy to find references to American popular music of that era. For an idea of what would have been heard around France & Germany during the WW1 era the following web site will be of help.
http://parlorsongs.com/insearch/fren...renchalley.asp
This article gives the best survey of popular music in France and Germany that I've seen. During the war years, popular music composition and distributon became quite subdued for obvious reasons in Europe. Nevertheless, what they would have heard would have been European versions of tin pan alley, ragtime, updated walzes and drinking songs plus the usual patriotic and military fare.
I'm sorry I couldn't find a source for recordings of this music, although the web site does have a few samples.
I did find a few web sites where the military music is available and the emphasis tends to be more toward WW2. If interested, I can forward these to you.
Best Regards, FliegerJG1