There are a couple of possibilities for this, depending on your equipment. If you have an AV receiver set up for surround sound (5.1 or 7.1), but you don't actually have a center speaker, it's important to set up your receiver's speaker configuration so that it knows to send center channel audio to the left and right speakers instead. Otherwise, center channel audio will be simply discarded. Many movies will only play dialog in the center channel, and some games never play dialog in the center; some of either can also play dialog across all speakers, so this problem may not be immediately apparent. In other words, just because one game or movie has audible dialog and another does not, doesn't mean that you don't have a problem with your AV receiver setup. This can only be fixed by setting up your receiver's speaker configuration correctly. Every system is different, so you'll need to consult the manual for your receiver to find out how to configure it.
Another case where dialog can be soft or inaudible is when using a TV or other device with 'dialog enhancement', 'spatial enhancement', or any other number of unique proprietary sound processing techniques. Many of these processing techniques were designed specifically to work with movies, which mainly play dialog out of center only; this is not all true for games where dialog is positional and rendered all around the player. For games it is best to disable any of these sound enhancement techniques; this will usually solve positional audio volume issues. For TV speaker use, it's almost always best to force the PS4 System into 2.0 channel mode, which will then render the center channel audio to the front speakers automatically.