Being able to communicate with Power Virtual Agents by voice would be a major development.
1. Being able to call Power Virtual Agents from a mobile phone or land line.
2. Being able to call Power Virtual Agents using an internet connection, through a native control to this effect in Power Apps (model, canvas, portal).
3. Being able to call Power Virtual Agents with Cortana.
4. Being able to call Power Virtual Agents through third party assistants.
When you think about it, the chat interface is the front-end, visually displaying a conversation between a human and a bot. The voice options would simply be an alternative front end. The experience for the bot maker would be as similar as possible.
Keep it simple, no code, citizen dev level. Everything native. Azure can be running the show behind the scenes, like everything else in the Power Platform, but this should be transparent to the maker.
Basically, speech recognition mapped to bot inputs and natural speech playback mapped to bot responses.