No Sound at all during calls on apps like Viber, Skype, Line, Tango etc

I bought my Gright HD 8i Tab a month ago and from the beginning there has been no sound when the call is attended on apps like viber, skype etc.... Normal phone calls are working flawlessly.... My Android version is 4.2.2 Jelly Bean(& it is not upgradable) Please help.......