[Zubersoft]

That isn't what batch import audio is designed to do. That feature was added because users requested that I provide a way to create songs containing only audio with a blank page. So it pulls in all the audio files, creates songs for them and adds the blank page. The feature is not designed to basically batch edit a series of songs and update them to add matching audio. If you had matching audio files in the directories when the PDFs or other files were first imported with Import->Local File, or Import->Batch Import, then the audio files would have been automatically adding to the songs (assuming you enabled "Automatically add matching audio" in the settings). 

Having said that, if you just use Import->Batch Import and you avoid duplicate songs, it will add matching audio for songs that already exist in your library. So you can achieve what you are looking for by using the batch import feature, just not the batch import audio feature.

Mike