Issue with sound

For the past two days, I've had three meetings where the participant sound is on at the beginning of the meeting and all of a sudden I do not hear the participant anymore even if they have changed nothing and their mic is still active. 

I've also noticed that the quality of their camera get worse once that happen. They seem to always have a full wi-fi connexion. In all cases, the participants was in France. 

Is that an issue that is happening to others?