I have found that the best thing is to run the clip once, I run mine as a questionnaire, in MediaLab, it won't show the visual but you will hear the sounds. Then without closing out of MediaLab when the clip is done immediately go to run your experiment. My clip plays every time visual and sound when I do this. I will usually just go back in and delete the file created from running the clip prior to the experiment when my subjects are done.
Hope this helps I know it's tedious but it's the only way I have found that works!