The Media URI refers to the location of the recording. It should be stored in the location defined in the Recorder policies in IA - Recorder. If the recording is not in the configured location, you get that error. To find where the recording is, search IR_RecordingMedia table in the DB for the RecordingID. There is a field called MediaURI that shows the location of the recording. If it's a screen recording, you'll likely see the field populated with 'scrrec://[username]@[machinename]...' which means it has not processed off of the user's machine. If it's an audio file you may see 'http://...' which means it has not processed off of the media server. If it's blank, either no policies applied to it, or the purge policy that applied is configured to purge only media (so it left the record in the db), or...something else. :)
The most common reasons I've seen for this is the disk space for the storage location is filled, there's a network issue transferring the recording from where ever it is, or the ScreenCaptureTransferU service is not running on the user's machine.