Thanks for reaching out, very good question. So, we implemented 3rd party AU (Audio Unit) audio plugins in a previous version of our app (1.4.x), but due to issues with syncing to these 3rd party AU's we removed the functionality until we can create a smooth user experience (the videos you've seen online, I'm guessing, were recorded on an earlier version of the app).
As of a couple weeks ago, we have discussed - and I've personally researched - how best to implement 3rd party AU's without causing app errors. This is a tricky issue since we aren't in control of how 3rd party AU's are developed, so we're trying to find a solid way to check for stability before loading 3rd party AU's - another tricky bit is to make sure we keep the audio in sync with the video clip, so there also needs to be an acceptable latency when using 3rd party AU's. But we're striving to implement this functionality as soon as we can!
As far as the Apple provided AU's goes, you can double-tap on an clip (that contains audio), in the timeline, then tap on the 'speaker' icon, which will present you all the Apple built AU's (AUHigh-pass filter, AUReverb3, AUDistortion, etc.) <- but even with these plug-ins we've discovered abnormal functionality (for example: sometimes the AUReverb3 causes audio to lag).
So we're also looking into providing stability, not only for 3rd party AU's, but also for the Apple AU's that come prebuilt into iOS.
Please keep us posted at firstname.lastname@example.org
with any specific AU issues you run into.