It is possible to get well under one tenth of the size of the original with comparative quality. Lower rates start occurring what is known as low signal to noise ratios. The less information extractable from a piece of data the higher the noise ratio. For example television pictures are made out of a high quality black and white picture with colour smeared on top. Colour is less important for a clear picture than a detailed monochrome image.
Telephone lines use information theory to fit voice information down the line.
The information the ear is said to not need is thrown away to fit more music in a smaller place. The secret to good compression is therefore being able to work out how to get the information out of the data recreate the data at the other end. Currently the most advanced way of doing this is to use fractal compression. Using heavy computation patterns are squeezed out of pictures and stored as templates which are then used to reconstitute the picture at the other end. This technique will even fill in gaps in a picture if a section is missing, although this may not work very well on things such as faces.