Why does your recorded or objective voice sound different to what you hear in your own head? When speaking, I hear my own voice very differently from how others do and from what it really is. The sound differs in tone, pitch, volume, etc. For example, recordings of my singing or speaking in foreign languages sounded a lot worse than what I heard in my own head.

Is that a popular phenomenon? Or is that a "syndrome", and which parts of sensation and perception are unusual? Last, is it modifiable?
Jens' answer is pretty much spot on, but misses the fact, remembered from my undergraduate lectures, that your ears actually partially 'turn off' when you speak (or chew), in what's called the stapedius reflex (wikipedia).
The most common reference I've seen for this is Møller (2000), which unfortunately is a book, but I'm sure more information could be found with a little digging.
Reference
Møller, Aage (2000). Hearing: It's Physiology and Pathophysiology (illustrated ed.). Academic Press. pp. 181–90.