Once you have the Arduino libraries installed you can try these examples!
All of our example code lives in the github repo at https://github.com/adafruit/Adafruit_NeoTrellisM4
Load this example to display a variety of colors and effects on the NeoPixels, good to test that they're all working and shining as expected. You need to have the Adafruit NeoPixel library installed first
Load this example to turn on NeoPixels whenever you press a button. Press again to turn it off. Good for checking button presses, elastomers and NeoPixels.
Your NeoTrellis M4 can act as a 32-button MIDI board. This example will send Note On and Note Off reports for every button when pressed and released.
The Audio library (originally by PJRC) allows the creation of waveforms and filters dynamically by the chip! You can try this out with the simple synth example, which will give you an octave each of four different voices. The playback is polyphonic, try pressing multiple buttons!
This example will take microphone input and then pipe it out the headphones. Simple but good for checking that your headset is wired correctly.
You can take audio input, then display it on the LEDs - here's an FFT example. You don't get a lot of bins but it can make for a neat audio effect!