[gasbag]

The reason for the phenomenon is that some large percentage of how you hear your own voice comes from bone conduction. In addition, the higher harmonics of your voice are more directional, which is to say "aimed away from your ears", and tend to be diminished when reflected back to you by the objects around you.

The end result of this is that your own voice, when recorded and played back to you, will generally sound less bassy and more harmonically rich than you expect it to.