Open the application and go to File > Settings > Audio and Video. Check the Automatically adjust my microphone box and select the Speaker audio and Windows default device as Speakers for application sounds. After that, click the Check your audio & video devices option and check the microphone, speaker and webcam. If there is a problem, reinstall the drivers and read the ooVoo troubleshooting guide by pressing No when you will be asked if the device is working well. Also, make sure that your friends do the same.