Hi,
I heard that Unity3D’s default audio is based on FMOD. Is that so? I am new to Unity so I do not know. But there is also plugin available at FMOD site for Unity integration. I am not sure now whether this plugin has anything extra to offer or is it the same as default Unity audio ? Also, my aim is to develop a sound based navigable environment. So I need some good audio settings (stereo, 3D etc).
Hi, to learn more about Unity features, you can visit the Manual (Audio Chapter) or check out the Editor feature overview.
For info on a specific Plugin, you should contact the Plugin’s authors, because they know best what it does.
Unity’s audio engine is easily capable of high quality 3D sound. Supporting up to 7.1 channels and as much voices as your hardware can handle, you will find it has enough power and flexibility for audio-navigation based games. 
Of course how you use certain functions is up to you, because it’s your game, you can create it as you like.
If you are not sure at one point, you can always google your problem or question and you will most likely end up here on Unity Answers or the Unity Forums.
That leads me to my final advice: Please don’t post a new question anytime you have one, if you google it first you will find it answered 90% of the time, because most forums reach back to at least 5 years. That does not include this question, just a tip for the future.
Here a link to the Frequently Asked Questions and the User Guide. Have a read of them at least once, so you can use this forum as efficient as possible. 
Have a nice day.