You can use one earbud at a time if you wish, but this does not give you a mono mix of the source, you will only hear ‘half’ of the stereo mix, so you will only be able to listen to either the Left or Right channel when you do this (depending on which earbud you have in). So, for example, if you were listening on the Left earbud to a song with guitar mixed only on the Left channel and drums mixed only on the Right channel, you would only hear guitar.