Unfortunately you have provided very little information about your issue.
On most devices, when audio is playing the volume for that audio can be adjusted. I believe with the introduction of Pie, Google changed the way audio is controlled. Prior to this when you pressed the vol buttons and the slider displayed there was an expand button that would show you all of your audio volume options. Since then you can only control volume for what is playing, i.e. On a call=ear piece, music or video=media, alarm sounding=alarm, so on and so on. By default just pressing the vol buttons with nothing playing adjusts media volume and other volume controls are only accessible though the settings menu. This is why I'm confused about your question because you appear to be asking for something that you should already have, even in older versions of Android, so you must be leaving something out???
What device do you have?
What version of Android are you using?
What app or apps are you using that do not allow you to change the volume while audio is playing?
Have you checked your audio settings to see if there is a setting or default that you can set to give you the control you are looking for?
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