There is nothing that allows you to control the volume of specific apps in the stock OS.
In some apps there could be an option in Settings to manually adjust the volume but that depends if the app developer implements it. This way it will adjust the volume of that specific app.
You could send feedback to a specific developer if they do not have it or jailbreak. If you jailbreak you loose your warranty:
While the jailbreaking process was ruled legal in 2010, Apple was very clear that doing so voids your warranty. This means that you could potentially lose out on a warranty fix for an unrelated issue because your iPhone is jailbroken.