Methods and apparatus to detect spillover in an audience monitoring system are disclosed. An example apparatus includes means for sampling first and second audio signals. The first audio signal is received from a source by a first microphone. The second audio signal is associated with the source and received by a second microphone. The second microphone is separated from the first microphone by a first distance. The example apparatus includes means for calculating a time delay between receipt of the first and second audio signals. The example apparatus includes means for determining whether the time delay is within a threshold time difference. The example apparatus includes means for retaining media associated with the source when the time delay is within the threshold time difference. The means for retaining is to ignore the media associated with the source when the time delay exceeds the threshold time difference.