Check your Flash Player settings to check if the sound is enabled. You should see a app called Flash Player settings on your Android main menu.

No enable sound option in flash settings...

When doing playback on your device, have you check its volume is the correct? Remember Android has three volume controls you have to check: Multimedia, Notifications and Alarms.

If it is OK, then maybe the issue is on the site. Which site are you visiting? If you are trying to watch videos on Youtube maybe you'd better visit m.youtube.com where you can use tablet's native media player to watch the video.

In my tablet I can't event start Flash. It is installed and browsers detect it but movies just don't play. Flash for mobiles is very unstable.