There's a plugin called Procrustes that I tried. It was recommended highly by some WinAmp users, but it's really difficult to find as it is no longer available through the WinAmp site, and the developer is nowhere to be found. Eventually I found the plugin here: http://users.hal-pc.org/~clement/Winamp/Modern%20Folk%20Dance%20Skin.html
- inside a zip file linked halfway down the page under the link name, "Security template". It works, but it's a pain to use once installed. You have to go to Tools > Options... > DSP Plugins and check the Procrustes box, whereupon a slider pops up that lets you change the playback speed. But the slider goes away when you close the Options dialog, and to get it back you have to navigate back to the plugin, and then un-check and re-check the box.
The request remains: MediaMonkey needs a simple, easy-to-access command to adjust playback speed. Even just a few buttons for 1.25x, 1.5x, and 2x would be adequate for me, but of course a slider would be better.