The internal operational state of wellhead equipment and downhole activity can be monitored with a sensor device mounted or mountable on the wellhead equipment. The sensor can include more than one vibration sensor, a sensor communications device, and a processor. The vibration sensor generates sensor signals in response to vibrations of the wellhead equipment caused by changes in the internal operating state of the wellhead equipment or downhole activity. The processor generates sensor data based on the generated electronic sensor signals. The sensor communication device transmits an electronic data signal for the sensor data via a communications network to a user device, which may be located remotely from the wellhead equipment. The user device can output a report including an audible or visible representation of the transmitted sensor data. In some embodiments, the sensor device can be retrofit to existing equipment and systems.