Default D802 Calling - Can't be heard properly by others

Hi folks,

I've just bought LG D802 in Romania ( COSMOTE ).

After first boot I updated via OTA to KK 4.4.2 and many people complained about hearing me very bad when I call them or they are calling me. They say that they hear me like I was talking from a cave or something....

I googled this issue and I already tried to uncheck the voice enhancement from the Call Settings, but the problem persisted, so I googled it deeper and found that I have to flash the moded to 12B or something like that so I downloaded the zip. The problem is that I don't know how to flash this zip file without having any SD Card (yes, I already rooted especially to do that).

So, I would be very thankful if someone could answer me to these simple questions:

1. How can I flash the zip file without SD Card (of course LG G2 doesn't support this by manufacturer) ? PLEASE EXPLAIN STEP BY STEP like I was stupid
2. Is there any other method to fix this problem without flashing ?

Thanks in advance!