Flash Player is just what the name says... a "player". It doesn't generate sound on its own and it has no settings for sound output. It makes sound based on browser settings, system settings, and HTML in pages with video.
Things to check:
1. Do you have sound elsewhere? (other applications)
2. Have you checked your physical connections? (at the PC and at the speakers)
3. is the volume muted in the browser window? (the webpage)