The symptoms (I should note that my host's sound works just fine) are one of several situations. In most configurations, when windows boots, there's a little red X over the loudspeaker icon in the tool-tray thingy. If I click on this, it launches some kind of diagnostic tool which ponders for a while, then says something on the lines of "I've installed a new driver, reboot" but when I reboot, the whole thing is the same. For some configurations of VB, I can get sound output, but still no input.
I've upgraded VirtualBox to the lastest version (as of today -- 5.1.22), and have upgraded several times over the life of this problem. Each time I have installed the corresponding guest additions. I have tried Intel, Soundblaster and HC-er,whatever hardware emulation, and also OSS, ALSA, and Pulse.
I can get *output* but only output using Pulse and the Intel hardware emulation. If I select ALSA and ... I forget which, but one of the hadware emulations, I get evil sound that's about half-speed, but also no input. With OSS, the boot process for the VM gets a VB message saying that no sound could be opened, and while windows thinks it's making sounds (volume controls show, that kind of thing) there's no output.
In the VM settings, audio shows as enabled, but of course there's no separation between input and output. When configured in Pulse/Intel mode, in the pulse volume control tool, it shows the VirtualBox / Windows 10 in the recording devices tab, and shows a moving VU meter (which is almost insulting, really!
TIA
Simon