What does it show if you test flash version? Find version
Does it show flash player is installed and working?
Boot into safe mode with Networking Safe Mode
In safe mode open any of the browsers, surf to youtube and try and play any video (you likely will not have sound). Do videos play? If so, try a clean start to see if they will play in normal mode with minimal services/startup items. Troubleshoot Application Conflicts by Performing a Clean Startup
If that also works, you have something conflicting. Follow the tute to enable items and track down what is interfering.