Note: This functionality is only present in sensors with "PDT" in the model name
The microphonic technology on the SSI sensors has an automatic gain control. It means that it is able to adapt to the environment and will filter out constant background noise. It remembers that information even if the sensor is power cycled. If the frequency of the noise changes, it will adapt to the new frequency.
The sensors also have an Automatic Set Back function. If sound is resetting the time delay of the sensor and no PIR is detected for 30 minutes, the sensor will revert the microphonic level to the low setting. If another 30 minutes passes with sound only, the sensor will then revert the microphonics to off and the sensor will then time out based on the time delay.
Typically, if the vent is the suspect that keeps the sensor in occupied mode, its not the sound from the vent. It could be blowing hot air which is triggering false PIR detection.