Adding audio only to notes which are missing sound

When populating audio for large decks which may already have some audio, you may want to skip notes which already have audio. You can achieve this by entering a filter string in the Anki browser. In this case, I want to find notes in the Mandarin deck which have an empty Sound field.
The following query works: deck:Mandarin Sound:

If your audio tag is inside a text field, you'll need to look for notes which don't have the mp3 string, and you can use the following query: deck:Mandarin -mp3 (show notes in the Mandarin deck which don't contain the text mp3).