Some systems reproduce waveforms with a small delay that varies by frequency--i.e. high frequencies are slightly delayed relative to low ones, or vice versa, so

Some systems reproduce waveforms with a small delay that varies by frequency--i.e. high frequencies are slightly delayed relative to low ones, or vice versa, so the frequencies are out of phase with each other, relative to the original. Phase isn't intrinsically audible (though the brain does use it in stereo imaging) but it could be indirectly perceivable when it interacts with other aspects of a system, especially nonlinear aspects.