Sound issues at vocal and piano studio

Hi all

I run a virtual music studio as part of my business.  Lately I am having issues with sound coming through from my students. 

I can not hear them sing or play, but can hear every back ground noise in their space.  I have the musician settings turned on as per Zoom instructions.  They use phones, tablets or computers and some of them can not be modified for sound settings such as Android and IOS devices.  

Any suggestions on how to fix this so I can run my lessons?

Thank you in advance