Which version of Android are you using? Is it rooted?
Might be a shot in the dark, but other than Subsonic, can you play any Flash videos at all on your browser? Or do they all show the black screen? Because if you're having a general Flash problem I might be able to help. A few months back I had a similar issue on my rooted G2, running CyanogenMod 7, where I would get sound but no video from any Flash videos, no matter which app I was playing them in (for example, as an embed in Dolphin Browser, or in the Pulse rss reader), and no matter which version of Flash I had downloaded from the Market. All I would see is a black screen, or maybe some glitchy and corrupted green and black screens.
It could be fixed by editing a file in the Flash plugin folder, for each app I was trying to use. You can use Terminal Emulator to input these commands:
1. su
This gives you appropriate permissions to get started.
2. cd data/data/
com.dolphin.browser/app_plugins/com.adobe.flashplayer/.macromedia/Flash_Player
This changes the directory to the Flash folder inside of your app. Notice the part in green, that's interchangeable with whichever app you were using. They will all have their Flash folder located with that same structure. If it's the Dolphin browser, you would use com.dolphin.browser or if it's the stock browser you would put com.android.browser or if it's Pulse you use com.alphonso.pulse, etc. (you can find out what each app is called, by the way, by entering the command "ls" before you do step 2. It will show a list of all files in that folder, including all app names)
3. chmod 000 config.data
Hell if I know what this does, I just know that it works.

Changes some kind of permission for the "config.data" file.
If you had typed everything exactly as written, then it would fix that Flash error that would occur with Cyanogenmod 7. Or who knows, maybe I typed all this up for nothing.
