Vanilla "Solid Inspector" [NOT 2] indicates the issues.
"Solid Inspector 2" will fix many issues, but you will need to fix some yourself manually.
At the expense of "boiling these cabbages more than once" [Irish saying]... I'll reiterate what a 'Solid' is...
A Manifold Solid [at least as far as SketchUp is concerned] has the following properties:-
It is a "Container" [i.e. a Group or Component-instance].
If you Select it and the 'Entity Info' top-bar says 'Solid' then it is a 'Solid' [BUT subject to the provisos at the end...]
It must contain only Geometry.
Geometry is defined as Edges and Faces [Curves/Arcs/Circles are made from Edges, Surfaces are made from Faces].
Although the non-interacting Guide-lines and Guide-points are also "allowed" [and probably would not affect an export to STL or similar], I'd still recommend you avoid including them.
So the 'exclusion' list includes nested Groups or Component-instances, Text, Dims etc...
[You example does contain some nested parts in the microphone]
There are further limitations on its 'Geometry'.
Every Edge must support two Faces - no fewer and no more.
This means that an Edge CANNOT:
be 'faceless'
have only one face - e.g. forming a ledge, shelf, hole-perimeter etc
be shared by more that two faces - e.g. supporting an internal partition face where it has three or more faces, or a shared edge between to volumes which would otherwise be considered a solid but they'd have four faces
There are 'CleanUp' plugins to remove coplanar or stray-edges etc.
You can use Xray mode and temporary Section-cuts to access the inside of these forms to tidy them up by Deleting unwanted edges/faces.
There are some other issues which might affect 'Solidity'...
The Faces of a Solid should be consistently oriented.
Use Monochrome mode...
Front-faces [Style-dependent, but usually off-white] to the outside, and back-faces [Style-dependent, but usually blue-gray] to the inside.
Once you have a Solid, then the Context-menu > Orient will fix this.
You can still have a SketchUp 'Solid' Container reported, which is not 3d-printable !
If its Geometry defines a 3d object that cannot exist in the real world it'll fail !
An form like this 'intersects' itself - like a magician who reaches through a solid wall to touch something on the other side.
To treble-check for this, you can take a Container and edit it, Select All and Context-menu > Intersect with Context.
If you exit the Edit and it's still a Solid in Entity Info all is OK.
If it is not then something was wrongs and you need to fix it, as outlined earlier...
