Is there any way to boost it? It seems to be louder on the rear of the phone.
I have a GT-I9505 Galaxy S4.
You can turn up the volume of a call on your phone by pressing the volume buttons while in a call.
Make sure that you are using the "earpiece" speaker setting instead of "speakerphone." (The latter would cause the volume to be louder in the back.)
See if you can find any debris that is obstructing the earpiece speaker and carefully remove it with a small tool of necessary. (This is unlikely to greatly impact the sound/quality of the speaker.)