They have a serious manufacturing defect commonly known as the “Audio IC” issue. Apple downplays it, but eventually it hits almost every 7 series.
Apple offered a free repair program for a while, but has since discontinued it.
The 7 series is pretty much “designed to fail.” We have seen flawless phones in Otterbox cases have this issue.
The 6 Plus also has defects on the Touch IC chip. We recommend the 6s or 8 series for customers.
If anyone reading this has a 7 or 7 Plus and is concerned about this issue, back up your phone on a regular basis at a minimum. Trade it in for an 8 or 8 Plus when you can. If you’re happy with your 7, you don’t need to go to the X/XS, but I would recommend stepping up to the 8.