The stereo sound glitch is a glitch in Pokémon X and Y. When sound is set to 'stereo' in the Nintendo 3DS system settings, the sound in the game plays as if it is in mono, even though the option selected in the system settings remains unchanged.
This glitch can be noticed by entering a wild Pokémon battle and listening to one of the channels. In mono and stereo all channels sound equally loud from the same speaker (unlike music from the World 1 map in Super Mario 3D Land, where it sounds different in stereo mode). In surround, one of the channels may sound quieter.