Sounds like a cool project!
Alright now I get it. Yes that is definitely possible with Wwise alone, but I'm not sure how the result will be if you want to have it "trigger" the sound, like playing from the beginning. Here's what I'd do.
1) Create a Sound SFX (let's call it - 'SFX 1') in the AM Hierarchy.
2) Add the Wwise Audio Input.
3) Make a new Audio Bus in the MM Hierarchy and add a Meter Effect to it.
4) Connect that Meter to a new RTPC.
* maybe add slew rate to the RTPC as well.
5) Make the same RTPC control the volume of the 'other' sound you want to play (let's call it - 'SFX 2').
6) Add both SFX 1 and SFX 2 to the soundcaster, and press play on both.
This should hopefully, make the microphone input level control how much you'll hear of the 'other' sound. However, you'll probably need a more length one or looping so it won't stop at some point.
Now if you want it to "Trigger", so like start from the beginning every time, you could set it to use virtual voice and then set the virtual voice to start from beginning.
Hope this helps!