You might remember when u were in school the teacher used to tell you to read aloud. This technique is basically used to ensure that u actually register what u are saying and dont miss anything. With maturity, the mind automatically internalises this voice, which is what u hear as the inner voice. This has a disadvantage in the fact that it slows down your reading speed, almost down to the speed with which u talk. U can try an experiment - while reading something, try to manipulate your inner voice to talk some gibberish - anything, say la-la-la...u might not be able to register clearly what u are reading, and this indicates to what extent u are dependent on this inner voice. With practice, u can actually make urself able to read (and register) stuff u read without the inner voice.This will increase ur reading speed. However, the depth of understanding u get will be inevitably reduced. U might notice that the voice is not present when u're not really attending to what u're reading,and u might have to read the same stuff again and again to actually register it. Sometimes it also dissapears during loud reading, which is because it is simply not required that time.
U can actually control ur "inner" voice to be present when reading some "heavy" stuff and read without it when doing some easy reading.
Happy Reading ;-)