Wow, so I finally figured this out. Hopefully it will help someone else who is unlucky enough to have the same problem.
OLD namespaces WERE as follows:
main project namespace: CompanyName.AdvancedAudioGui
assets project namespace: AdvancedAudioGui.Assets
When I added the x:Name attribute on an instance of a control in the main project that was defined in assets project, it would fail with the error:
The type or namespace name 'Assets' does not exist in the namespace 'OurCompanyName.AdvancedAudioGui' (are you missing an assembly reference?)
When I changed my namespaces in the assets project to more closely match the main project, the error went away.
New namespaces are as follows:
main project namespace: CompanyName.AdvancedAudioGui (same as before)
assets project: CompanyName.AdvancedAudioGui.Assets
The latter way is preferable anyway, but it appears that the assets namespace being a subset on the main project's namespace somehow confused the compiler when it was trying to parse the xaml namespaces.
Maybe someone knows why that was the case...?