Subsonic* is a media streaming server that is similar to Plex, but focuses on music rather than video.  This channel should allow Subsonic streams to be played through Plex.
 
The plugin is available from:
https://github.com/caveatsCode/subsonic.bundle

To install:
Download the subsonic.bundle folder from github.  Copy the folder into your Plex plugins folder (you may need to change the name from 'subsonic.bundle-master' to 'subsonic.bundle').  Restart the Plex Media Server.
When you open the channel in the Plex client of your choice, you will see two options- 'Artists' and 'Preferences'.
First, go to 'Preferences' to enter your Subsonic server address, username and password.
Then, you goto 'Artists' to start playing music.
You should not need to edit any files to get the channel to work- all setup is done within the Plex client.

To do list:
Improve track loading time (decrease buffer size?)
Search function
Scrobble to last.fm
Shuffle
Other navigation options besides Artist (such as genre, decade)
 
If you test this channel, please let me know what works and what doesn't work for your setup.
 
*You need Subsonic Premium to use this channel.  It currently costs $1 per month from the Subsonic website (http://www.subsonic.org/pages/premium.jsp).