Translating Windows system audio in real time — driverless, with no virtual cable

I build Voxis, an open-source Windows app that translates whatever your system is playing — a video, a game, the other side of a call — and plays the translation back as spoken voice, a few seconds behind the speaker. No subtitles, no virtual audio cable, no bot joining your meeting. The "no virtual cable" part is the bit worth writing about. Almost every system-audio tool on Windows tells you to install VB-CABLE or VoiceMeeter, or to drop a bot into your call. Voxis doesn't, for incoming audio. This post is how that capture engine works, and the sharp edges I hit building it in Python. I'll be specific about what's hard and honest about what's not mine to fix. The goal Read the exact audio the user is hearing — the post-mix system output — at 16 kHz mono, and do it without installing anything. Then stream it to a translation model and play the result back, all while the original keeps playing underneath. Three constraints fall out of that: Driverless. If it needs a reboot and a driver, it's not zero-setup. No self-feedback. The app plays translated audio into the same system mix it's capturing . Naively, it would capture its own voice and translate the translation. That has to be impossible by construction, not patched with an echo gate. Realtime-safe. Capture can't stall. If the downstream VAD or garbage collector hiccups, the WASAPI ring buffer must not overflow. WASAPI process-loopback: capturing the mix, minus yourself Windows 10 version 2004 added the ApplicationLoopback API — a way to activate an IAudioClient in loopback mode scoped to a process tree, either including only that tree or excluding it. Excluding our own process tree is exactly what constraint #2 needs: the captured mix is everything the user hears, with Voxis's own output removed. You don't get this client from the normal IMMDeviceEnumerator path. You activate it by name through ActivateAudioInterfaceAsync , passing the loopback parameters in a PROPVARIANT carrying a BLOB : params = AUDIOCLIENT_