The old audio waterfall plugin for SDR# seems to be no longer available for download anywhere (it may have gone out of date and is no longer compatible with the latest versions of SDR#). Alan Duffy decided to write his own version of the audio waterfall plugin and make it available for download. An audio waterfall shows the demodulated audio in waterfall form, essentially creating an audio spectrum analyzer. This can be useful for understanding the demodulated frequency structure of a signal.
To install the plugin simply download the dll from his website and place it in the SDR# folder. Then open plugins.xml file with a text editor, and add the magicline specified on his page.
Note that for us Chrome detected this file as malicious, but this is a false alarm as Chrome does this often with unknown .dll files. To recover the file we had to go to the Chrome menu -> Downloads, then select "Recover File" to download the file. (If you still have problems with the download then check out the comments as some users have kindly mirrored it). (File was moved to a trusted site so this problem shouldn't occur anymore)