How to get if a specific sound is still playing in a channel?

Is there some kind of sound ID I can get when playing a sound that I can use to check if a channel is still playing that specific sound?

I know how to get if ANY sound is playing in a channel but if one sound has finished then that channel might be in use for another sound, because I look for any free channel everytime playing a sound.

The only way to do this is to have a custom system that tracks when a sound has been created (and its name) and when a sound has been released. You can then query this system to return what is currently in play. Take a look at the “sound played” and “sound stopped” event callbacks of the documentation:

https://www.fmod.com/resources/documentation-api?version=2.1&page=studio-api-eventinstance.html#fmod_studio_event_callback_type