I just found that WMP11 can’t select which audio stream is played from a DVB-S m2t (AVCHD) file. So I turned to Linux to figure out how to fix a Windows bug as usual!
Turns out the fix is easy and takes about 1m20s:
ffmpeg -i infile.m2t -acodec copy -vcodec copy -map 0:0 -map 0:4 outfile.m2t
The 0:0 is the video track and 0:4 is the English audio track we want to keep; 0:1/2/3 are the voiceover audio and subtitle tracks we want to remove.