Actually, there are Youtube videos that play without Flash by using HTML5: it started 3 years ago. The fact that the video played silently showed that either Flash was installed or the video didn't use it.
Did you pay, Kishan? If not, you have a demo disk. CentOS would be the same as Red Hat, but that too expects you to get your own copy of Flash. Unless you particularly need Red Hat, get a friendly distro that has everything you need ready to use: Mint is a very popular one and suitable for beginners. You should find the sound works.
The first thing to do when sound doesn't work is to look for a sound configuration tool in the menu, run it, and make sure that volume isn't set to 0%. It seems silly, but I've seen that happen.