I am thinking about entering the field of engineering where I research and work on amplifier, speaker, subwoofer designs, among others.

I do not know what math classes to take (Currently Freshman in HS) in order to best suit myself for this type of engineering (audio/acoustics engineering). Thanks for help. :)

If you're going to major in engineering, then calculus and differential equations will most likely be required.

As a freshman in High School my advice would be to do well in the core math HS curriculum - algebra, geometry, trig (or algebra 2 as it is often called these days). It would be really beneficial to have an AP level calculus course by the time you graduate from HS. That way when you get into an engineering college you could place out of the first freshman level calculus class, and get into more advanced math areas such as linear algebra and differential equations.