I’m sure better learning material will be forthcoming after 4.16 is released, but for reference in the meantime, here’s how I did it (I’m sure there are better ways):
1. The new audio engine won’t be available until 4.16, so download and compile a recent Master or Promoted branch of UE4 from GitHub is available now in 4.16 Preview 1 in the Epic launcher:
2. Enable the new audio engine by adding the applicable line to any platform-specific engine config files for your project (see 4.16 preview release notes for syntax):
3. Under Editor Preferences -> General -> Experimental -> Audio, enable the “Show Audio Mixer Data” option This is no longer necessary!
4. Enable the new audio plugins:
5. Profit! Now the fun starts… add a ModularSynth component to a blueprint (the main Pawn in this case). All parameters can be left default, but in this example I’ve added a reference to a Source Effect Preset Chain asset which contains some additional effects:
6. On the Pawn’s BeginPlay event, define some basic parameters for the synth. You only need a minimum of 1 oscillator, but in this example I’ve defined 3 for a nice, thick sound (note individual oscillators indexed 0, 1, and 2 for later reference). And finally “start” the synth so that it can be used (simply setting the component to Auto-Activate doesn’t seem sufficient):
7. On any forward input, if a note isn’t already triggered, then start one. This note will continue until a command is sent to stop the note.
In this example, I have defined an array of integer semitones [0,4,5] for the notes C, E, and F, from which I’ll randomly select a note to play each time the accelerator is pressed. Also notice that the NoteOn function references a Note parameter indexed 0 (zero), so we could potentially be starting and stopping many separate notes (up to the Voice Count defined for the Modular Synth component) with separate calls to NoteOn/NoteOff, each referenced by their index (although the Note parameter appears to be a float, so surely I’m missing something).
NB: This example actually sends 3 potentially different random notes (a new random integer is pulled for each reference) to each of the 3 oscillators, due to a logic error. (To send the same note to all 3, I should have called RandomIntegerInRange only once and assigned it to a temp variable…) But I like the variably discordant sound of how this turned out 
8. To raise the lowpass filter frequency based on speed: on each Tick event, get the current velocity and apply the ratio of that versus max velocity to the ModularSynth’s filter frequency.
In this example, the variable SpeedCamMax is a number slightly higher than the max velocity the pawn can reach in-game (due to wind resistance), so the filter frequency will be set between 50 Hz and 12 kHz depending on current velocity:
9. And finally, stop the note whenever the accelerator is released (this is checked on every tick).
Note that in this example, it’s only turning off 1 note: the Note parameter 0 (zero), which we referenced when starting the note. In the example video this single note sounds like a chord due to me erroneously sending different random semitones to each oscillator on each NoteOn
Apologies for the spaghetti code, hopefully this helps clear things up for someone!
I’d seen references to presets, but didn’t quite get how to use them. Thanks for the tip!