Share your experience!
If i call with skype or viber my sound is very low (if use secondary mic back of the phone it normally not low sound)
If i call regular call microphone it normally both no problem
Why and how to fix it??
Since it works just fine, as far as I understood, when using the regular calling function it's a bit tricky to say what might be causing the issue.
Have you tried re-installing the applications it appears with to see if that might solve it? Otherwise it might be a good idea to contact the developers of the applications to see if they have any further ideas. 
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I have the same issue. This happens in the exact same way in applications that are independently developed (Skype, Viber, etc.) . It is therefore more likely a telephone/system fault rather that a problem with a single application.
Please let us know what to do to solve this problem, because not being able to use any sort of voip calls on a smartphone is a pretty serious issue.
I have the same issue and I googled, its been happening with other Xperia users, I have Xperia S and it never happened before, just from the last update the problem has started. During any viber call I have to switch on the speaker to listen clearly which feels still too low and after using viber once, the xperia goes to some kind of silent mode without lowering down the sound indicators. If you check the volume bars, they show full volume but moving them right left does not give any sound. The only way to get your phone volume fully functional is to restart the phone. However viber works perfectly fine on my friends android and apple phones.
Are there any app tests done before floating software upgrades? I personally think that phone software/firmware is not compatible with new viber application.
Please wake up Sony and Petter and test these phones. You are not even relasing the new or one yr old JellyBean.
Urgently help since its not saving my money anymore. Please do not give me update links, I have done it all. Please show us test reports if possible. Thanks!
ok i found the solution myself, and its working fine for me now. Do the following:
Go to settings--> Applications-->All-->Click Viber-->Click "Move to Phone"
I think viber when installed in internal storage of Xperia S, the problem occurs. I moved a lot of applications to internal storage cuz there is a natural lag in Xperia S software. Although there shouldnt be a problem even with the internal storage but Sony didnt bother to diagnose or test and blindly blamed app vendors.
I hope it works for you, try the same with skype and other VoIP apps.