Here is a very common use case I have: I listen to music on Spotify while going through my Facebook feed. Sometimes there's a video that I want to see in full screen. When I tap the video, music playback stops. What I want is to watch the video without any sound and keep my music playing. Is that possible? Phone: Galaxy S8
It is not normally possible unless the apps have been written to release the speaker.
Android offers the concept of "audio focus", a set of APIs that can cooperatively be utilized by third-party apps so that only one app can hold the focus at a time. Apps can choose whether or not their audio should halt or "duck" whenever another app takes over the audio focus. Because Google left it up to developers to decide how to handle things when the audio focus is lost, there's a lot of inconsistency in how apps behave when another app takes over audio focus.
Fortunately, there's a new mod for rooted phones called "App Volume Control" that seeks to solve this.
If you are willing to root your phone, then you can try Per App Volume Control