Responsive theme will adapt itself to different devices (like mobile and tablets).
If you use flashplayer - people on mobile and tablets will not be able to watch those videos since those devices do not support Adobe Flash. So you'll need a script to detect if flash is present/or device and to use HTML5 player for devices not supporting flash and flashplayer for the ones that do. To use HTML5 player you should encode clips as MP4 (and not as FLV).
Hope it helps...
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