Viber mic volume is too low topic

I am wondering if anyone using viber and facing this issue or not.

currently when trying to make\ receive calls on viber, the other side complains that the volume is too low

I had tested this against also my Galaxy S5, the fire phone still have very low mic volume, so I can barely hear.

I suspect the problem might be that Viber is picking the wrong mic (if fire have multiple mics), or just gain issue.