Methods and apparatus to generate threshold values for state detection are disclosed. An example apparatus includes a threshold generator to maintain, in memory, a collection of audio data elements representative of sound levels present in an environment at respective times when ON-indications are detected for a monitored media device. The threshold generator to replace, in response to detection of a next ON-indication in the environment, one of the audio data elements in the memory with a new audio data element representing a sound level in the environment at a time of the next ON-indication. A selector to select, for a value of a threshold condition of a state detection analysis, one of the audio data elements in the memory corresponding to a lowest sound level, at least one of the threshold generator or the selector implemented by at least one of a processor or hardware.