sig-alternate class uses a peculiar1 way to set the document fonts.
In particular, the
\texttt macro selects the font family
aett (which belongs to a definitely obsolete package) at size 9pt, independent of context. So you have two problems.
Use a boldface version of the Typewriter font.
Increase the font size in section titles.
Solution to problem 1
Solution to problem 2
Due to how the class is written, you won't be able to make a table of contents if a command such as
\sectt appears in a section title. But this would happen also with
\sqrt and other seemingly innocuous commands
The editors of the conference to which you're submitting the paper may not be happy with these changes.
1 The adjective "peculiar" should be read as "blatantly wrong".