1. How to disable notification sound?
Very simple. Start editing your profile. You will get an option to choose the notification tone. Set it 'off''.
2. How to avoid unwanted private message?
This is easy. Start editing your profile. There you can disable messages from non-friends, disable all kind of private chats and also disable 'Private Video/Audio chats'.
3. Why cannot my friends see the group I just created?
Created groups are hidden by default. You need to invite your friends using the group menu. When you create a group, you become the owner of that group, and you can set the role of other members of your group.
4. I left my own group by mistake. How can I get it back?
If your group has more than 10 active members, you can ask the admin for help. If your group has only few members, you are advised to create a new one.