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 There are, though, possibly tweaks with jailbreak, but the one you mentioned is pretty outdated and will either just not work or cause bigger problems.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.

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.

(Source)

As of iOS/iPadOS 14, you have a new possibility to do that with the built-in app Shortcuts. Here for iPadOS 14:

1. In Shortcuts, create a new automation with App as the trigger
2. Set the option to Is Opened
3. Click on the Choose button and select the app you want
4. Click on Next (upper-right corner)
5. Click on Add Action
6. Select the item Scripting
7. In the list, select Set Volume
8. Click on the value (by default 50%) to change it
9. Click on Next (upper-right corner)
10. De-select Ask Before Running and click Done

Now every time the app is frontmost, the volume will be set automatically to the specified value.

Caveats:

• Usually, the automation is executed immediately when the app starts up but it may take a few seconds sometimes.
• Whenever the app becomes frontmost again, the automation is executed again.