Audio quality issues might be caused by the audio device that another caller is using. Ask others on the call to help check the following:
If they are using their webcam's microphone or computer speakers, ask them to try using a different audio device.
Make sure the device they are using is optimized for Skype for Business communications software. For a list of optimized devices, see Phones and Devices for Skype for Business.
Have the presenter mute the participant who has the malfunctioning device.