I've installed Zoiper 5.3.8 on Windows 10 x64 (build 1909) and in the Audio tab under Settings, the input/output device/speaker options all show as default and the dropdown doesn't function, which is to say clicking it does not show my audio devices (nothing pops up, clicking is a no-op).
Clicking the play button on the output device plays no sound. If it's doing anything at all, it's not using the Windows default audio device.
I tried choosing both 32-bit and 64-bit options from the installer with no difference. I also tried explicitly running as Administrator and again, no change.
"Allow apps to access your microphone" is enabled.
I have a few different audio interfaces: RME Babyface Pro (primary interface), FastTrack C400, and a Corsair Virtuoso headset. I do a lot with audio day-to-day (including working in Reaper and Davinci Resolve), and no other Windows applications have presented me problems using any of my audio devices.
Any thoughts? I'm a happy Zoiper Premium user on Android, and buying Zoiper for Windows seemed like a natural fit now that I, as I'm sure many others here, will expect to be working more from home. Alas, I'm stuck right out of the gate.