I am having the same problem with the international version of the S4 active. As with lots of others the problem does not exist with the backcover removed or when using phonespeaker or headset.
But I also found that there is also no echo, when just removing the microSD card and putting back on the cover and not using phonespeaker or headset.
It seems, that there is some kind of electrical interference as long as the SD is installed or some kind of incompatibility with SD cards? I just found this, hence did not try yet with diffeent vendors from SD cards or putting a "dummy" in it, just to see, if the issue also exists, if there is no actual electrical contact.
I also heard from other unoffical sources, that Samsung is planning to get this fixed with a new firmware?
Cheers,
Timo