All you need to do to use the visualizer and filters on your NeoTrellis M4 is to plug it into USB, download the firmware .uf2 file below, place your NeoTrellis into bootloader mode. Bootloader mode is entered by double-clicking the reset button on the back, and drag the firmware file onto the TRELSM4BOOT drive that shows up.
If you plug your NeoTrellis in and the drive shows up as CIRCUITPY, that means it is in CircuitPython mode. No Worries, press the reset button until you see TRELSM4BOOT which is the flash drive to place your .uf2 on. If you were to copy the .uf2 to CIRCUITPY, it would not run, hence ensuring the drive is TRELM4BOOT.
Once you've flashed the firmware, here's how to use it:
Plug the output of a music player, such as a phone, computer, mp3 player, or Walkman, into the audio jack terminal block adapter
Then, connect headphones or powered speakers to the NeoTrellis M4's built in TRRS 3.5mm jack
Power the NeoTrellis M4 over USB
Play music on your player
The NeoTrellis M4 will automatically apply the low pass filter, and start visualizing the music in the lower section of the NeoPixel lit buttons -- it should have a muffled sound since the mid and upper frequencies are being filtered out
Press and hold the bottom left button (NeoTrellis M4 in vertical orientation, USB jack facing left) to hear the pure, unfiltered audio, and note how the visualization changes
Let go of the bottom left button and press the one above it to switch to a band pass filter
For more advanced hacking, if you want to tweak the Arduino code, you can head to the GitHub repo to have a look!