I assume more people are going to run into this problem with any of the Subsonic streaming apps on Android, so I'll post what I know here. This affected my Samsung Galaxy S4, but it sounds like any Kit Kat devices with MicroSD card slots are also affected.
The latest Android 4.4/Kit Kat update DISABLES the ability for "unofficial" apps not pre-installed by your wireless carrier, Google, or Samsung to write to the MicroSD card. For a detailed technical explanation of what changed, read this:
http://www.androidpolice.com/2014/02/17 ... ting-them/
Subsonic apps will install and work correctly if you leave them with their default settings to use your device's internal storage. However, if you specify your external MicroSD card (/storage/extSdCard on the S4) as your storage for any Subsonic Android apps, it will stop working with the following symptoms:
-The app will operate normally and will connect to your server and list the artists/albums as usual.
-If you updated your device from Jelly Bean...stuff you've already streamed and is available in offline mode will play normally.
-As soon as you play a new file that isn't already saved to the MicroSD, it will display album art, but the song won't play and either stops completely (the official Subsonic app) or repeatedly count from 0:01 to 0:04 without playing anything (Ultrasonic).
-Nothing is saved to the music or artwork folders on your MicroSD card.
Unfortunately, the only way around this is to root your device and restore the permissions for any apps to write to your MicroSD card with a free app from the Google Play store called SDFix. If you're looking for a method to root your device, I highly recommend the XDA-developers forum. It gets harder to root with every new device and Android update, but they have the knowledge and expertise to guide you in the right direction.
I hope this helps someone. I spent three days troubleshooting the problem until I had another issue copying files from the internal memory to the MicroSD with ES File Explorer and started researching the specific issue.
Needless to say, this is completely dumb. I'm not sure why Google and/or Samsung would do this, as it renders your MicroSD card useless for anything but what your carrier, Google, or Samsung deems appropriate.