We (Viber) cannot officially recommend this.
In some devices, Viber cannot "handle" the microphone (since we don't fully support that device), causing Viber not to recognize the phone's microphone properly. That is probably the reason that Viber cannot transmit your sound to the other side. We discussed this issue in detail with our users in this thread on XDA Forums: http://forum.xda-developers.com/show...5#post25865775
Please note that in that thread, a workaround was suggested *by a user* (and not officially by us) that seemed to have solved the problem for Galaxy S+S2 users.