Sometimes someone has the sound settings so loud that the microphone registers the sound coming from the speakers as someone speaking, resulting in sending the sound back over the microphone. When this happens, the other parties will hear an echo or find the sound output very hollow.
To solve this, simply ask the person with the loud settings to adjust the volume. This can be done from within the videocall by the bar in the picture below. The bar automatically hides when there has not been any mouse activity for a few moments so you may have to move your mouse cursor over the video screen to make it appear.