Well I have some good news and some bad news. The good news is that I found the air leak; it was coming from the drain plug closest to the front of the pump. I took the plug out and made sure the threads were clean and the gasket was seated well. I then put the plug back in, went to tighten it so it wouldn't leak again, and I snapped the head of the plastic drain plug off. I was able to "fix" it for the night so I could still do some vacuuming of the pool and I could let the pump run as normal by sticking some duct seal over the reminder of the plug. And I did some searching around to find suggestions for removing the broken one, like using a hot screwdriver to melt a notch with. Where do I get a replacement? I don't want to remove what is left until I have a replacement since the duct seal seems to be working pretty well.
A slightly more concerning thing is that when I took the cover off the pump basket, I found an O-ring in the basket! I checked all the places I could think of for o-rings; namely the connection into the pump, the connection out of the pump, and the connection into the filter, but they're all present. I'm not sure where it could have come from...maybe it fell out of the pool guy's pocket and into the basket.... Any other suggestions on that mystery?