Methods, apparatus, systems and articles of manufacture to determine headphone adjustment for portable people meter listening to encoded audio streams are disclosed. An example system disclosed herein includes meter data analyzer circuitry to determine an audience estimate for streaming media based on media data measured by a media meter, the media meter to measure the streaming media based on an identification of the streaming media in ambient audio collected by a microphone of the media meter, and data analyzer circuitry to calculate at least one headphone adjustment factor based on a first determined proportion of an audience that listens to streaming media via headphones and a second determined proportion of the audience that listens to streaming media without headphones.