It would be around/above 20KHz, not below; our hearing is theoretically ~20Hz-20KHz (though almost noone starts out actually able to hear that high and the high

It would be around/above 20KHz, not below; our hearing is theoretically ~20Hz-20KHz (though almost noone starts out actually able to hear that high and the high frequencies decrease with age). AD/DA converters will also apply a low pass filter below Nyquist to prevent aliasing - assuming they are operating at 44.1/48KHz (and even if they are spec'd higher there's no real reason they'd always be running the higher rate), Nyquist is about 21KHz, though filtering lower than that is beneficial as it allows them to employ a less steep slope on (very steep filter slopes can create artifacting of their own). The small speaker set up in your average phone would be quite capable of outputting 18-19KHz, which is still outside most people's hearing range, and unless you're particularly attuned to high frequency audio you might not notice or work out what it was anyway.