Q. Should I need to change the silencer for to get sound - Bullet 350?

Yes, if you are having bs4 model than you can change your silencer but don't go with the free flow silencer go with the K-Indori silence or silencer having filters in it. And be careful because silencer change is against traffic rules so you might get caught by police and they will fine you some money and in some areas your bike can be sealed.

Yes to enhance the sound you need to change the silencer but with doing this change the warranty of your bike will be void. Also you can face the strict action of traffic police & more over they can cease your bike.

Yes if wish for more thump after market exhaust is only choice.