Stereo Speakers - definition
From the very beginning more or less every phone have speakers in-built for external audio output, phone companies provides a single speaker with mono channel in phone. But very few mobile manufacturer who are highly concerned about media in the devices, provides dual speaker & make the external audio output signals a stereo. A mono and stereo are totally different. Mono is single channel audio signals but stereos are dual channel audio signal and far clearer & which supports Hi-Fi.