This clashes with cmbright by redefining bold math to use bx rather than sb. As you can see there are three basic commands and they can be nested to get combined effects. In certain cases it may be desirable to include normal text within an equation.
The added benefit here is that you can have better control over the font formatting rather than the standard text achieved with text.
Since you already had the fonts set up the way you wanted them suppress these redefinitions by adding the line usefontthemeprofessionalfonts to your preamble. First arguments to section move around which makes section one of the so-called fragile commands. With these you cant for instance italicize and make a text bold at the same time. The boldmath declaration switches to a bold math italic font.