The problem is that rtppcmupay
does not support stereo:
$ gst-inspect rtppcmupay
...
Capabilities:
audio/x-mulaw
channels: 1
rate: 8000
....
You can try some other codec (e.g. vorbis):
$ gst-launch alsasrc \
! 'audio/x-raw-int,channels=2' \
! audioconvert \
! vorbisenc \
! rtpvorbispay \
! udpsink host=127.0.0.1 port=5555