Alsa is used for audio playback.

NX does some initialization of audio:

• amixer sset PCM 144
• other magic to get it actually working?

People have had mixed results with Alsa initialization. One person only had success by running NX:

Saving and restoring alsa device state isn't enough to initialize with from boot. The NX binary is built with miniaudio inside of it so perhaps it does some external hardware initialization of the speaker? Couldn't get miniaudio in a custom binary to work without initializing by running NX. Maybe there's a way to coax out the miniaudio debug logs from a running NX instance.

Another person found that it worked out-of-the-box:

Copying an audio file (in my case, a 44.1kHz mono WAV file) onto the SD card and then playing it with aplay worked even with NX disabled. alsamixer is preinstalled and works for setting the volume.