Works fine for me. Couple things about sound:
- The first time you playback anything on the site, it can take a while for the sound to start. This is because the browser needs to load up whatever helper app it uses to playback the MIDI. Every subsequent playback should be speedy
- Playback can fail if too much data is being asked for. In some browsers, if you ask for a 32-bar groove to loop 16 times, it might not play because the helper app can't handle that much MIDI data. When in doubt, set the loop to 0 or 1.
The bulk of what is known is listed in the sound help page:
If some lessons are playing back and others aren't, I would be surprised if the difference is because a lesson is newer.