Audio signals are alternating. The voltage corresponds to the sound pressure picked up by the microphone, which typically looks more or less like a bunch of sinusoids.
As an EE student nearing a bachelor's, I haven't seen all that much use for more abstract mathematical topics in my classes. I would say it is much more important to be very comfortable with the more basic, concrete topics that will show up all the time: algebra, ODEs and PDEs, (vector) calculus, Fourier/Laplace transforms, linear algebra, probability. An EE specializing in the physics side of things may wind up using abstract algebra or other higher math, but that is a pretty small percentage of engineers.
Racket is very easy to get going with, you just install it and use the IDE, which gives you the text editor/code area and REPL together like emacs/slime. Scheme is a smaller language. For example, there is no flet/labels, you just use (define) for nested functions. It's a good way to get going with a Lisp and see if you want to take on a heavier language like CL or Clojure.