Methods, apparatus, systems and articles of manufacture (e.g., physical storage media) to detect media watermarks in magnetic field data are disclosed herein. Example media monitoring apparatus disclosed herein include means for transforming magnitude values of magnetic field data to a frequency domain to determine transformed magnetic field data, the magnetic field data associated with a first sampling rate, the magnetic field data obtained from a sensor. Disclosed example media monitoring apparatus also include means for detecting an audio watermark in a portion of the transformed magnetic field data associated with a first frequency, the audio watermark encoded in an audio signal, the audio watermark to have a frequency component associated with a second frequency different from the first frequency, the first frequency to be aliased relative to the second frequency based on the first sampling rate.