My vocal recordings are sounding somewhat "boxy". I wouldn't say they are muddy or unclear. They come across sounding like they were recorded in a small box, damp and dead.
And you have built:
Recording booth: 4x4 corner of a 10x8 room. The actual area has 3 walls covered in Audio mute absorption sheets (Sound Proofing, Soundproofing Material: Lowest Prices - Audi mute Soundproofing) and a few layers covering the front and creating a top at about 7'.
Don't think you need to look much further...
It sounds as though you've completely overdamped the corner, and that you are probably getting the rest of the room into the back of the microphone - which
will make it sound boxy. Anything representing 'booth' conditions in a room as small as that isn't going to help you at all to record vocals in there. First rule of any room treatment - 20% of the treatment does 80% of the work. And as a consequence, you really don't want to go much further than that. In a room like this, you'd be better served by a couple of single acoustic tiles at mic level on the wall, and an
SE electronics reflexion filter with its back facing into the room. Anything else in a room that small is going to cause you trouble, unless you turn the entire room into your vocal booth, treated appropriately.