> Does a film listing not having any of these abbreviations default to German audio?
In my experience, yes. The default is German without subtitles. I have seen the OV and OmU abbreviations a lot of times, but most showtimes don't have anything listed which will mean German.
OV = Original Version
OmU = Originalfassung mit Untertitel (original with German subtitles)
DF = Deutsche Fassung (German version)
OmeU = Originalversion mit englischsprachigen Untertiteln (original with English subtitles)