The Viber voice is barely heard when the speaker is on.

[Mhers]

Hi,
My Phone is Galaxy S4 mini
Model number: GT-I9192
Android version: 4.2.2
Viber Version 5.7.1
While I'm using Viber with speaker on, the voice is barely heard.
But phone line is normal with speaker on.

If I use older Phone Galaxy S2 ,
Model number: GT-I9100,
Android version: 4.1.2
Viber Version 5.7.1
The Viber speaker voice is normal.

Could you please tell me how to solve this problem?

-Mher