Thanks ,
Note that Steam Audio plugin depends fundamentally on new capabilities in the new audio engine, so you’ll need to run with that enabled if you want to see Steam Audio editor extensions and features. The same is true of the synthesis plugin or Sound Utilities plugin. I wrote code to specifically hide new functionality from sound waves, etc, if the audio engine isn’t enabled. Note that the Synthesis plugin will be where source and submix effects are written, and new synthesizers. The Sound Utilities plugin will be where I implement utility-like features and BP-type applications (e.g. utilities for constructing interactive music and dynamic ambience, etc).
See the release notes for specifics on how to enable the new audio engine (you have to either launch with -audiomixer or modify an engine .ini file).
Note that the new Steam Audio plugin integrates deeply with the audio pipeline as we were able to collaborate on how to better integrate Steam Audio (and also other third party audio plugins) to the UE4 audio workflow so it might not be exactly the same as it was in the past. For example, adding the ability for Steam to define their own settings data layout, but still be able to plug those settings into the standard UE4 spatialization definition implementation pipeline. We also streamlined getting access to source and listener data for the plugin.
Unfortunately there’s not a huge amount of documentation (or barely any) for any of the new audio engine stuff as we’ve been almost exclusively focused on making sure what is there is working (specifically for 4.16 audio engine features). We’re also not fully finished with all the platforms so don’t want to enable things without there being full platform support.
RE: Android/iOS/Switch support for the new audio mixer. I didn’t write these backends so there are some issues I need to fix before it’s fully utilizable. Apologies for those excited about android audio finally having full feature support in UE4. It’s close, but needs some love and attention. I’m literally working on Android right now to attempt to fix some of the obvious buffer underrun issues (I suspect it’s because the mix function is being called synchronous to the audio output stream buffer callback).
Thanks everybody for being patient with us as we transition to using a totally new audio engine back end.
In fact, since there’s not a lot of documentation currently, I’d be super grateful for anybody who posts some stuff to these forums/wikis with how-to’s as you discover things 
We’ll be trying to post blogs/tutorials/live-streams in the coming weeks on how-to’s on new audio engine systems shouldn’t change by 4.17 (e.g. how to use the synths, DSP effects, etc).