I uploaded an audio clip from my mac to dropbox in mp3, wave, and aac formats. When I try to add media in Rush, it sees none of these files. I tried to restart Rush, but not luck. It sees other audio files in the dropbox folder, but not the one's I recently uploaded.
Also, just to test, I tried to upload one of the audio files from DB that I did not want, the app just hung up at 'loading' and never loaded the audio.
In trying to replicate the problem you are describing, I opened a project in Rush, tapped on the blue add button, navigated to my project folder on DropBox, selected the audio file (wav) then tapped the blue add button at the bottom of the screen. I was able to see the wav files in DB and add them to the sequence.
Can you check the Sort at the bottom of the panel to see if you are excluding audio for some reason?