gst-launch -v \
 filesrc location=~/media/tripod.avi ! decodebin name=d1 \
 filesrc location=~/media/duck.mpg ! decodebin name=d2 \
  d1. ! audioconvert ! audioscale ! volume ! audio/x-raw-int,rate=48000,channels=2 ! adder name=add \
  d2. ! audioconvert ! audioscale ! volume volume=0.75 ! audio/x-raw-int,rate=48000,channels=2 ! add. \
 { d2. ! queue ! ffmpegcolorspace ! videoscale ! ximagesink } \
 { d1. ! queue ! ffmpegcolorspace ! videoscale ! ximagesink } \
{  add. ! audio/x-raw-int,rate=48000,channels=2 ! queue ! audioscale ! alsasink  }
