The notification sounds is on, by default. However, it turns it self off after sometime. I cannot manipulate it from the volume adjustment bar. I can only turn it on if I turn the unit off first then on after a minute or so.
Are you sure you did not update your Android Version?
The unit is on automatic update. I am not aware of any updates before that happened.
There you go, it might be the updates. Sometimes bugs are caused by an incomplete or failed update. So try to do a Soft reset, else Hard Reset. You have your warranty? Make use of it. This is likely a System problem since S4 does not have any settings to change the volume keys (except if you are playing music).
Android application has its well developed and useful functionality that makes your task easy and error free. Questionable link REDACTED - SS
Yes it does, the problem is if the Auto Updates are installed on his S4. It is advisable to install the latest updates for android but the cost are bugs and other errors which is currently common nowadays.