For anyone interested in how my experience went:
I subscribed to the Audio last night. I then downloaded Firefox (which had been recommended, and because mlb.com states that it won't work with Safari) and that program linked above by another reader (the flip4mac thing). At first I was unable to get any audio to come through even though the page would load. I closed everything down and reopened Safari, figuring I'd try it there. The page failed to load properly; however, I did get audio. I listened to the last few innings of the Cub game this way.
All went well enough, with two exceptions: 1) the sound is tinny and low-band, but it's clear and 2) for some reason, the stream "skipped" from the top of the 8th back to the top of the 7th, and then played on as if nothing had happened until the game ended. Strange.
When the game was over, I closed Safari and reopened Firefox. I logged on to the Audio and clicked on the Boston game to test whether or not I'd get any sound now. It came through perfectly, page graphics and all.
Overall, if it continues working like it did last night, it'll be worth the $15/year. The sound isn't perfect (we listen through our stereo receiver, which adds a bit more bass to it, fattens it up a bit), but it's clear, which is more than I can say about our reception of WGN in the house. We live right across the lake, in Michigan, and in the car WGN comes in fine. At home it's a different story.
My biggest Q about all of this is simple: why can't mlb.com join the 21st century and give us high-quality, streaming access to the games, especially if we're paying for the service? The whole setup they're using seems held together with duct tape and silly string. There are many, much smaller sites, that stream live rock shows and radio and other things WAY more easily and with MUCH higher quality. And they all do it for free. Of course, what am I saying? We're talking the MLB here.