As part of PTE's process, before you take the PTE exam, you will be given a chance to test your headphones and microphone. Test everything thoroughly. If you happen to find any slight issues with the PTE staff as this will affect your exam scores.
One of the common issues is the microphone. At times the microphones are faulty and when you test them the playback sound is not too clear, catching some weird sounds or just sounding off in general.
Another tip is to make sure you hear everything clearly so adjust the volume of the headphones as needed. Also, make sure it fits securely in your head so it's comfortable when you're taking the exam.