This is complicated due to the microphone input component. The old VOIP talker API is not utilized widely and is definitely an older system. You’d need to write some C++ code to attach a source effect to the VOIP talker output.
We use the VOIP system in EOS (Epic Online Services) for Fortnite. I implemented some gameplay code that allows us to attach a DSP source effect chain on the voice audio so players can hear processed audio, so it’s possible.
You could check out the Microphone Capture Component but that is not hooked up to VOIP and was implemented as a means to get captured audio into Sequencer (the Sequencer Take Recorder uses it under the hood) – it’s not really designed for the use-case you’re talking about.
More broadly speaking, the way to arbitrarily route audio around is either through Audio Buses (which is essentially an audio buffer wrapped by an asset that you can use as a handle) or through Source Buses (which are audio buses turned into sound sources so you can hear them). Source Buses have an implicit internal audio bus if one is not supplied to them by the user. Source Buses allow you to control and play audio routed to it from other sound sources as another sound source (i.e. via audio components, etc). In this way, they can be thought of as 3D submixes. We utilize source buses quite a bit in Fortnite for a variety of different applications.