After I'd had my S4 for a few weeks, I started having an intermittent problem with audio cutting out on phone calls. I would not be able to hear the other party unless I put the phone in Speaker mode (where it would work fine). [Note also that if headphones are plugged in, audio works fine.] I googled the issue and saw that previous Samsung phones had had similar problems. People offered various suggestions for fixes (including turning off noise reduction, lowering the volume, etc.), and nothing worked. Finally, I ran across a post somewhere where the problem had gone away after the user loaded the Skype app and made a test call. I did this, and although the Skype call didn't go through (I was calling my landline phone), I haven't had the problem since. (Knock on wood.) [Addendum: I enabled noise reduction, and it cut out again. I'll see whether disabling it keeps it working.][Addendum #2: Even with noise reduction disabled, the problem cropped up again. It's starting to look like a hardware problem. Regardless, I'm still experimenting. Latest efforts: remove sound control from Power Toggles app, lower in-call volume to below 50%, and experiment with the various "Personalize Call Sound" options.]