Gee guys, this thread back from the dead. I sold that condo well over a year ago. Anyways, I ended up carpeting the return box (carpet aired out for months), then I wedged up a USPS cardboard box at the front of the rectangular cutout (at the unit intake area right under the filter). This in effect made the air come in and travel around the USPS shipping box, reducing the noise significantly. The return box had plenty of room, so doing this didn't starve the AC of air at all.
The carpeting is able to be replaced by removing the glued on pieces and cutting new carpet to fit. That didn't take long at all. I did that right before I sold the unit.
Putting in new outlets was outta the question for such a small condo. Putting in an additional register would have looked downright horrible and would have devalued my condo. So, you can't just say "put in new ducts and registers." Sure, if I wanted to make my place look butt ugly I suppose I could have.
The size of the unit I purchased was the smallest Carrier makes. I could have maybe gotten a smaller unit by some other company, but I wanted Carrier.
So everything went honky dory, except for the fact that I took a big hit on the sale of the unit after the real estate bubble burst.