depois de vários testes acho que encontrei algo que me satisfaz, acho que ainda pode melhorar, mas na hora vou usar isso:
//DESKTOP RECORDING WITH WEBCAM WITH SOUND
gst-launch-1.0 --gst-debug=3 \
ximagesrc \
! queue max-size-buffers=0 max-size-time=0 max-size-bytes=0 \
! videomixer name=mix \
! videoconvert \
! videorate \
! vp8enc min_quantizer=10 max_quantizer=10 cpu-used=2 deadline=1000000 threads=2 \
! mux. pulsesrc \
! audioconvert \
! vorbisenc \
! queue max-size-buffers=0 max-size-time=0 max-size-bytes=0 \
! mux. webmmux name=mux \
! filesink location=screencast.webm v4l2src do-timestamp=true \
! video/x-raw,format=YUY2,width=640, height=480, interlaced=false, pixel-aspect-ratio=1/1, framerate=30/1 \
! queue max-size-buffers=0 max-size-time=0 max-size-bytes=0 \
! mix.