Control volume of individual apps on iOS I would like to independently adjust the volume of Google Music and an interval timing app. The only app I see is VolumePanel but you need to jail break your phone. Is there anything available for the stock OS? 

Also, this is not a duplicate of this question, as that solution only applies to Mac OS.
Unfortunately no, the only volumes that could be changed separately are media volume and ringer volume, which is also alarm volume. 
There are, though, possibly tweaks with jailbreak, but the one you mentioned is pretty outdated and will either just not work or cause bigger problems. Search among tweaks released or updated starting at least from March 2018.