Create main.c

Pipeline:

filesrc location=./input.webm ! matroskademux name=demux \
    demux.audio_0 ! queue ! mux.audio_0 \
    demux.video_0 ! queue ! mux.video_0 \
        matroskamux name=mux ! queue ! output-selector name=selector \
            selector.src_0 ! fakesink \
            selector.src_1 ! filesink location=./result_x.webm  \
P.S.
output-selector default path: src_0 -> fakesink, when interrupt => switch to src_! -> filesink