...Port Audio is new to me, I google the Port Audio but I can't find anything to read about (I vaguely remember that I saw Port Audio around CryEngine V somewhere, but don't remember where)
PortAudio is a low level audio renderer. You'll find corresponding documentation here. http://www.portaudio.com/
While we do release the engine source code with a super basic implementation we do not officially support it hence not mentioning it in official channels. It's rather something experimental that audio code savvy people are invited to play around with.
I'm really new to sound engine, so I don't really understand what SDL Mixer/Port Audio can't do compare to what FMOD can do.
Fmod and Wwise are professional audio middlewares that are used across many AAA type game productions and other non-game applications. They offer a rich feature set and designer UI for a streamlined workflow and data abstraction. SDL_mixer and PortAudio on the other hand are super simple implementations that offer a limited set of functionality. The SDL_mixer implementation currently allows for 2D/3D playback and distance based attenuation while PortAudio in its experimental stage only allows for 2D playback.
About CRI ADX2, it was brought up as part of the big audio middlewares on this planet and to emphasize the fact that it could as well be neatly implemented to CRYENGINE. We hope to be able to spend some time on that in the future but don't have concrete plans to that regard, btw the same applies to The Miles Sound System.
But can I sum it up like this?: SDL Mixer/Port Audio can't do special sound effect, it can only play exactly what the sound file has and can do 3D sound, also it can play multiple sound, background sound,... at the same time?
Pretty much yes except for the current experimental stage of the PortAudio implementation cannot playback 3D sounds.