They are IPropertyValue objects.

If you have this:

auto props = AudioEncodingProperties::CreateAac(48'000, 2, 128'000);
auto prop = props.Properties().Lookup(MF_MT_AUDIO_SAMPLES_PER_SECOND); // Mfapi.h

You can get the value behind like this:

auto value = prop.as<Windows::Foundation::IPropertyValue>().GetUInt32();

Or this (C++/WinRT):

auto value = winrt::unbox_value<UINT32>(prop);

And to get an IMFMediaType reference from that, you can use MFCreateMediaTypeFromProperties like this, here with C++/WinRT:

com_ptr<IMFMediaType> type;
winrt::check_hresult(MFCreateMediaTypeFromProperties(
    props.as<IUnknown>().get(),
    type.put()));