Play speed is too fast with qml sink

[cfd new]

I made a simple pipelinegst-launch-1.0 filesrc location=test.mkv ! matroskademux ! h264parse ! vah264dec ! vapostproc ! audovideosinkit works fine in normal speed from command line and in a gstreamer C++ code.
But the play speed is very fast when I coded it in my GUI app with qml sink. The pipeline is the same except the sink is different.What can I do to set proper play rate?
   Joe 
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