Methods and apparatus to detect spillover in an audience monitoring system are disclosed. An example apparatus includes an audio sample selector to select first audio samples of a first audio signal received by a first microphone, the first audio signal associated with media; an offset selector to select first offset samples corresponding to a second audio signal received by a second microphone, the second audio signal associated with media, the first offset samples offset from the first audio samples by a first offset value; a cluster analyzer to, if a first count of occurrences of the first offset value satisfies a count threshold, accept the first offset value into a cluster; a weighted averager to calculate a weighted average of a second offset value and the first offset value in the cluster; a direction determiner to determine an origin direction of the media based on the weighted average; and a code reader to, when the origin direction is within a threshold angle, monitor audio codes embedded in the media.