what have you added to the setting Podlove | Podcast Settings | Media | Upload Location? It should be a stable directory, in which Podlove can find the media files by their slug. The latter is defined in Add New Episode | Episode Media File Slug.
And this is where I’m not sure whether Dropbox is a suitable back-end. Don’t they generate some random download URL for each file you upload? You may have to let Auphonic send the file to another service that is designed for public downloading.
Hope this helps. I you have more questions, please don’t hesitate to write back