arduinoFFT | Fast Fourier Transform for Arduino

I am writing code that uses a library with no default constructor (there is, but it's soon to be deprecated, and throws warnings), and I'd like to use it inside a class. Now, I am aware that there are several answers for this in general, but none seem to answer my particular variant.

Here's why: The library's constructor takes a pointer to two arrays that are used for input and output. I need to allocate memory for those arrays, and I have to do this before I call this library's constructor.

I thus need an alternative to constructor lists that will allow me to control the order, or I need to know how to allocate memory for objects inside a constructor list.

Here is the relevant code:

My class:

I have created a really basic FFT visualizer using a Teensy microcontroller, a display panel, and a pair of headphone jacks. I used kosme's FFT library for Arduino: https://github.com/kosme/arduinoFFT

Analog audio flows into the headphone input and to a junction where the microcontroller samples it. That junction is also connected to an audio out jack so that audio can be passed to some speakers.

This is all fine and good, but currently I'm only sampling the left audio channel. Any time music is stereo separated, the visualization cannot account for any sound on the right channel. I want to rectify this but I'm not sure whether I should start with hardware or software.

Is there a circuit I should build to mix the left and right audio channels? I figure I could do something like so:

But I'm pretty sure that my schematic is misguided. I included bias voltage to try and DC couple the audio signal so that it will properly ride over the diodes. Making sure that the output matches the input is important to me though.

Or maybe should this best be approached in software? Should I instead just be sampling both channels separately and then doing some math to combine them?

I am working on a pedometer application and I am running a real-time fft on accelerometer data where I use the arduinoFFT (kosme) library on an Arduino 101.

So let's say my sampling frequency is at 100Hz. I am interested in the up an down motion of the body which matches walking or running frequencies at around 1-4Hz. Many of the libraries including the one I am using seem to have a compute magnitude function. This effectively gives me half the number of bins used number of amplitudes (if I was inputting 64 samples, it will give me 32 amplitudes).

So my question is what frequencies does these amplitudes correspond to? In the example code the library provides, the frequencies are calculated as follows: