Hi @Stoic ,
Your Bevel (Amount) is too large for such a small model (I Hid one Face and disabled Face Orientation and Bevel, too better see what’s going on):

With the Bevel enabled you get this, so-called Z-fighting, where two Faces are (partially) in the same place:

With Face Orientation enabled, you get that red ‘line’:

Now, as for the original question of the Speaker Material, you appear to have been able to fix that.