The nether ambient sounds since they were added have been missing a variety of sounds in the 5 biomes that are in the nether, at least in each biome there are a number of +20 ambient sounds.
In the Java version it has categories of nether ambient sounds which are mood, loop additions and loop.

Here is a link from the Minecraft Gamepedia about ambient sounds:

https://minecraft.gamepedia.com/Ambience

Steps to Reproduce:

1. Create a Nether Portal.
2. Enter to the Nether Portal.
3. Stay in a nether biome for several minutes.

Observed Results:

The nether ambient sounds play between 2 to 5 minutes.

Expected Results:

The nether ambient sounds should be played constantly and non-stop.