Perhaps fungus gnat larvae?

Bagged soils like that are high in organic matter and have ingredients that enhance moisture retention are notorious for being infested with fungus gnats. I remember using a Dr. Earth product (I think it was a different product, but one that was also touted as being high in organic matter and good at holding moisture--maybe an older version of your product?) in outdoor containers, and I had a horrible fungus gnat problem. Have you seen little black gnats flying or crawling around? They'll sometimes fly right into your face because they're attracted to CO2.

Yes, they are flying all around. It looks like these gnats are the culprit. Great catch @Jean and @gorbelly! This article got me up to speed:

http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn7448.html

Had I read this sooner, I would have tried Bacillus thuringiensis israelensis or Steinernema feltiae. However, I already started a chemical solution this morning: pyrethrins. So, I'll see how that plays out. I will post the results. Thank you for your help!

You can use Bti as well, which is the Bt that is used on mosquitoes, which are in the same subfamily of flies. Gnatrol is a powder that you mix with water. People also water in crumbled up Mosquito Dunks or Mosquito Bits, which are also Bti based.

I would actually use a Bti product instead and drench the soil weekly with it. It will kill the larvae. IIRC, pyrethrins will kill adults on contact, but it's not a very effective way to eliminate the pest. You might consider repotting into a different medium as well.

Hi all,

Here are the results.... Per @gorbelly, I used Bti. After one week, I wanted to dig into the soil and confirm the larvae disappeared. However, the root system had become so dense that doing so would have broken many roots. So, I resigned to looking for indirect evidence. The results... gnats were still buzzing around and a new critter had moved-in (see attached photos). About 15 of them popped out from the mulch when I watered. I don't know what they are. The presence of gnats told me that larvae persisted, so I did another Bti treatment. Additionally, in order to eradicate the gnats and this new visitor, I applied pyrethrins. One week later, I didn't see signs of the new pest, but gnats still existed. So, I applied more Bti (very little). The following week brings us to today. I still see gnats. I guess I will do some more Bti. Anyway, thanks for your help =).

They may be in other houseplants as well. I once had fungus gnats in some potting soil, and the gnats spread to some orchids on another floor of the house, so treating the plants upstairs didn't get rid of the gnats because they weren't limited to those plants.