I know this question is a bit old, but I have been using Master Audio for a while now and I love it. The only problem I have is that sometimes I use other assets that explicitly use Unity's built-in system and you can't just rip out that code and replace with Master Audio so I end up having a hybrid solution where some things are controlled by Master Audio and some things aren't. However, as long as you hook other Audio Sources into the same Mixer that Master Audio controls then you can still adjust volume globally for them.
What I like most about Master Audio is the ability to have playlists, sound groups with variations, randomly play sounds in groups, and so forth. It also has audio occlusion for more realistic sounds in enclosed environments, but I haven't had much experience in using that yet.
For any professional sound, I strongly advise Wwise.
The integration with Unity is easy, you just have to set a few line of code in order to work (bascially "play sound", "get x value" "set state" and "set switch"), you can randomize pitch, levels, stereo (usefull for ambiances), manage your sound pool, just need one audiosource per object, easy optimization, and much more.
Every huge studio (Ubisoft, Io Interactive, Blizzard, etc...) as well as indie (The Game Bakers, Playdead, and so on...) are using that.
Depending of what you want to achieve, but sounds standards are really high now (even for small indie game).
ps : I'm not working for the company, I'm just a Sound Designer using it, and, compares to the unity' sound engine, it's like prehistoric age and today's world.
ps 2 : I've started a thread HERE to talk about it.
The software is free, but as a limitation of 200 sounds (and that is not a lot).
However, you need a license to sell your game. I don't have all the information about it, if your game is free, will the license will be free as well, or whatever, but they are open to discussion, so, it's pretty much up to you and what your game economic system is (free, free to play, etc...) Best way to know that is to contact them. (and you can check the licensing FAQ here : www.audiokinetic.com/licensing/faq/ )
As always, it's a matter of what your want your game to be. If the game is simple enough (let's say a 2D sidescroller with a jump and a few enemy) Unity is fine (although nothing is really simple ). But if you're going to the full Open world, Mocaped animations with ik rig and huge range of enemy, do not underestimate your sound, otherwise the game would feel unbalanced.
But afterall, I'm just a Sound Designer preaching for is own parish
Edit : And seeing what Devistute is doing with is Nocturnal game, I couldn't advise a bad sound