End of my tether- ongoing audio issues

We pay for our son to have Maths and English lessons online every day. Sporadically, the sound both ways drops out. His tutor and we can't hear any sound- we still have video, no sound. We have tried changing laptops, switching zoom accounts, checked the connection, sat him nearer the modem and both myself and my husband have been able to successfully complete other Zoom meetings with no issues- and his Tutor who runs a successful tuition business online states that only happens during the calls he makes to our son. 

Appreciate any advice or suggestions?!

TIA