Now, there is a possible solution to this problem. Why possible? Well, because it is still only an Indiegogo campaign which will need to accumulate $60K USD in the next month in order to see the light of day. The project, or rather the case, is called Fuze.
Not only does the case add a 3.5mm jack while still having a lightning port, but it also has an additional battery. The iPhone 7 version has 2100 mAh capacity and the iPhone 7 Plus one comes with 3600 mAh.