My code (https://github.com/raphlinus/synthesizer-io , just cargo run, see here for the code that sets up cpal playback) fails on my Windows 7 laptop with FormatNotSupported error.
I did some debugging, and the IsFormatSupported call is returning S_FALSE and the closest match the same as the request but with dwChannelMask filled in as 3 rather than 0. If I locally patch so that dwChannelMask in the request is initialized to 3, it works.