Chiming in here. Audio is something a lot of our customers are interested in, and I already have one friend working on using Flutter for video. Though I don't have any target date for when I could contribute to some Flutter audio code, I have a feeling one of the backers will get it going pretty quickly. And I've actually ordered a microphone and amplifier board so I have a chance of doing audio at some point. I'd love to make a Flutter walkie-talkie demo so anyone can quickly bring up audio with Flutter. I've got a Logitech headset with only about 30ft range, so even in my tiny apartment it can't reach the kitchen from my computer in the office! I'm hoping I will get the time to upgrade it to Flutter in the not too distant future.
That said, you won't use the USB here. Flutter can only be a USB device, not a host. However, you can hook the microphone and speaker amplifier directly up to Flutter's input and output header. I don't have a tutorial for that yet, but if you look around for using an Arduino with speakers and a microphone, there should probably be some good information. Flutter is pretty much the same as any other Arduino, though it's much faster than most Arduino boards so it can do some nice stuff. It also runs at 3.3 volts not 5 volts, so some interface boards may not work with Flutter without additional circuitry.
Should be a good project though!