What effect does my metal retainer have on my singing voice?
It should not have any affect on the vocal quality. It might, however, affect your articulation until you become accustomed to it.
I think my retainer question is different than the one I just read. My retainer is for my upper teeth and covers half of my upper mouth. The question I have is does this type of retainer prevent tones from entering my masque?
There is certainly an adjustment to make and it will alter the play of the sound with the hard palate. It does not, however, prevent tones from entering the masque. Many singers do remove the retainer while they are singing because they do not want to develop compensatory techniques that they will then have to adjust when they no longer need a retainer.