Been testing the audio #streaming from #Mixxx for tomorrow and it looks ogg Vorbis encoding is somewhat buggy. Don't have time to debug, but the solution is simple: take it outside of Mixxx - #DarkIce, a jack-aware command-line audio streaming client for #IceCast written almost 20 years ago. Rock solid!
Looks like simultaneous recording of Mixxx's master-out to #FLAC works ok.