As a long time Audacious user (since version 2.1) in Linux and Windows, from time to time I write here to report some issues. These are the latest I found:
1) The single instance in Windows works in the sense that if you have opened Audacious and want to reproduce another file, it opens in the same instance. And that's why Audacious is my default music player in Windows. However, a problem arises if you select different music files (in Windows Explorer) and try to open all of them (i.e, after selecting the files, I use the context menu and select "Open in Audacious", or double click if Audacious is the default player). The normal behavior in other players is that a temporary playlist is added and all files selected appear in that list. But when I use Audacious, multiple instances are opened, one for each file selected.
Thanks for your attention!