My wife and I both have S4 phones bought at the same time and each of them are experiencing issues. (both freeze and don't always ring...)
My phone freezes often and it is incredibly frustrating!
If I answer a call, they can hear me but I can't hear anything... so I have to tell them to call back after I reboot the phone.
When it is in 'locked up/frozen' mode notifications or phone won't ring.
If I try to play music, it may actually work after several reboots and normally ~10 min of screwing around with it.... but it may only play one or two songs before hanging again.
I read about similar issues so I backed up my data, reset my phone to factory settings and restored my backup. I also moved my music from the SD card and unmounted it. I turned off animations.
No change.... it's crap on a stick. When a $15 mp3 player is more reliable than an expensive phone there's a problem!