As described also in this answer, when docking controls in a form the order in which you add them to the form matters; i.e. when adding multiple controls that all dock to the top, the last control that gets added will be the topmost.
So, because you add your toolstrip programmatically and your menu strip via designer, the designer code gets executed first thus having the menu strip always at the bottom.
I think there are three ways out of the dilemma:
First approach
As Hans Passant pointed out, the easiest way to get things into the right order would be to simply call
stereoBar.BringToFront();
right after you added it to the forms' controls.
Second approach
To circumvent this you could also also add the menu strip programmatically and do this after you added the tool strip
.
Third approach
Another way out may be to add another container to the form (Panel or groupbox for example) via designer that also docks to the top that simply just functions as a placeholder where you add your tool strip to (so you do not add to the form directly anymore)