About two months ago it was mentioned in a nether update beta changelog that nether ambiance sounds were added, but for some reason, only a small minority of what java has (the ones that play rarely). However, there are a lot of missing ones, namely the sounds that play on a loop, all the time. 

How to reproduce:

Just go to the nether. You'll notice it's completely lacking any ambient sounds (except for the rare, occational ones). 

Why this needs fixing: The ambient sounds add so much atmosphere to the nether and it's sad that the majority of players won't be able to experience them at launch (if the bug doesn't get fixed).