Hm, the lack of braille output is a bit of a concern. I've made some changes to the display output in recent versions, what screen reader are you using and does your braille display work otherwise with other programs? The braille output in brushtone also isn't enabled by default, you can turn it on and cycle through the different display modes by pressing F6 and it will display pixels on the current row to the right of the cursor.
As CAE_Jones mentioned the Sonifier is based on Peter Meijers [The vOICe], and you can use that to help get a better feel for it. It works by converting the whole image into a single sound, or a region if your using the select tool in BrushTone, which is why you only ever hear one sound regardless of where the cursor is. Each pixel in the image is represented by a tone in the sound being played, the pitch of each tone determines the vertical position of each pixel with a higher pitch being higher up on the screen, and a lower pitch being lower on the screen. The time it takes to play determines each pixels horizontal position on the screen from left to right, and the volume represents how bright or dark the pixel is. By default Brushtone converts images to greyscale for sonifying, though it does have filters for the red, green, and blue channels of an image. While it can take some getting used to, its one of the few ways of getting a general overall view of an area through sound. Although perhaps I should also look into implementing some similar tutorials and guides for the Sonifier, or providing additional links and information for The vOICe in BrushTone.