I have updated my page on Moodle Audio Tools to reflect the current situation as of February 2009.
The situation today is much simpler than two or even one year ago, as Nanogong is the clear choice for doing this today.
Release 3 of Nanogong in August 2008 was a distinct improvement, especially integrating with Moodle.
The adoption of the Speex codec has improved the quality of the recording, which was one of its main weaknesses. And its integration with the Moodle rich text editor means you can record and play back sounds in any rich text box in a course, not just in the provided assignment module.
That's not to say there aren't some issues - with Gradebook integration of the assignment module, and occasional stability issues, but it has seen continual improvement over the last two years. So good job, Gong Project!
There have been no major improvements in the core gradebook for a long time, since version 1.5 at least. In the meantime, Michael Penny and Jeff Graham's 
