Almost all the questions you ask yourself while developing your site with Silence

How to display the submenus similar to this website presentation ?

You just need to add a auto-nav block with in the options panel :

Sub Pages to Display set to Display All
Sub-Page Levels set to Display all

And.. Last but not least, choose as 'custom template' the 'Silence Nav'. It will enable the mobile detection and other cool stuffs.

Why the menu does not fit on my mobile, as on this site, in combo?

You just need to select 'Silence Nav' as Custom Template for the auto-nav block.

Where are the sliders, tabs and accordions promised as included in the theme?

They are actually included in the "bundle". Maybe you have not seen the offer on concrete5.org and purchased the theme only. It does not matter :-), send me a message via concrete5.org, I will do the necessary for you to grant license for the four add-ons