Replaced parts, sound working off and on

I recently replaced most of the parts on my iPhone 6 Plus. The sound on it keeps coming and going. One minute it works, the next it doesn't. If I use a music app such as Pandora, the sound works fine. If I make a call or if someone calls or text, sometimes the sound works and sometimes it doesn't (on both the earpiece and on speaker mode). The lock sound and key clicks are the same, working off and on. I originally thought it was a bad connection, but would think while using the music app, the sound would come and go as well, but there is never an issue when listening to music.

The parts that I haven't replaced and are original iPhone parts are the home button, logic board, battery & speaker.