The list is created when the endpoint is first built, but formats will only show up there if the driver succeeded a format negotiation for that format (via the “set proposed format” property, or via a data range intersection.) You can repro the issue by: 1. Uninstall the device 2. net stop audioendpointbuilder 3. Reinstall the device 4. Set breakpoints on your format negotiation entry points 5. Net start audioendpointbuilder If that doesn’t do it, play a test tone.