If you want to search for text (i.e. what is being said) inside an audio stream you would have to process it with some kind of speech recognition algorithm and store the text as meta data associated with the files. For video you could also do text recognition for text inside the video. Evernote already does this for text inside image files, but has no support for audio as far as I know.
Something similar is possible when using audio to search for audio. I don't know the details of these algorithms, but I'm guessing they involve some kind of frequency analysis. Shazam is using this kind of technology to identify songs based on audio clips.
Here are some Wikipedia articles that may be useful:
- Speech recognition
- Fast Fourier transform
- Frequency analysis (frequency spectrum)
- Optical character recognition (OCR)
