It's not necessarily a function of the speaker but of the low-pass filter on the output. Unfiltered samples played back at 48 kHz will mirror the audio content

It's not necessarily a function of the speaker but of the low-pass filter on the output. Unfiltered samples played back at 48 kHz will mirror the audio content at 0-24 kHz to 48-24 kHz, and repeat that pattern at every 48 kHz multiple. (This is not quite exact because DACs emit step functions, not delta functions, and slew rate comes into play, but it is a rough approximation.) This happens even if the sampled signal is a sine wave. So, any output stage after a DAC necessarily incorporates a low-pass filter to eliminate the aliasing artifacts. A cheap low-pass filter will allow these aliasing artifacts through. You can simulate this e.g. by digitally upsampling a, say, 8 kHz sampled signal to 48 kHz without any interpolation.