What free sound engines are u using alternatively to included to cocso2d-x?
FMOD - is cool, but it is not free - Free version limited to 1 game on 1 platform in 1 year. Seems you still need to pay if using FMOD plugin API and FMOD Studio itself. http://www.fmod.org/download/
Superpowered - something new, but integration flow not clear http://superpowered.com/
Other alternatives?
I’m using SDL and SDL_Mixer. I’m quite happy with the results too.
https://www.libsdl.org/download-2.0.php
https://www.libsdl.org/projects/SDL_mixer/
Which is still cheap compared to the 30-40% share Google or Apple takes 
Can you go into details? What’s the issue?
I guess you are only interested in free stuff? If so, I’m only aware of OpenAL.
Non-free middleware:
https://www.audiokinetic.com, http://www.radgametools.com/miles.htm