Using FFmpeg loop filter makes audio disappear
The loop option can't be used if you're ingesting multiple streams via the filter.

Use

ffmpeg -re -f lavfi -i "movie=filename=ShortLoop.mpg:loop=0"
       -re -f lavfi -i "amovie=filename=ShortLoop.mpg:loop=0"
       -vf setpts=N/FRAME_RATE/TB -af asetpts=N/SR/TB     test1.mpg