Esther Dyson recently raised the issue of why people talk so loudly when they talk into Skype or mobile phones. There have been many theories advanced as to why this happens. At first many people attributed this behavior to general rudeness or a feeling of self-importance that came with being one of the first users of an expensive new toy, but it turns out there are more subtle processes at work.
Don Norman and Dave Farber have pointed out that mobile phones usually lack sidetone - the feedback of your own voice that provides the reassurance that you are talking loud enough. There is speculation that the mobile phone manufacturers left it out because the circuitry would be too bulky or expensive and that it was difficult to make it work in the noisy, outdoor environment were the phones were likely to be used. According to some reports the Motorola RAZR and other, modern phones do incorporate this feature. It would be interesting to see if users of such devices were perceived as being more polite.
Probably not, according to some research done by Andrew Monk at the University of York and reported on by Jakob Nielsen. Monk compared how people responded to being exposed to two person conversations vs. hearing only one side of a cell phone call. Even though Monk kept the volume the same (as measured by a sound level meter), the subjects reported that the cell phone conversation was louder. I've always thought this was because people were annoyed at not being able to eavesdrop on the entire conversation, but Nielsen has a more intriguing theory: that people are better at tuning out the drone of a two-way conversation, perhaps on some unconscious level.
Of course, as Esther has pointed out, "sometimes there just really is a crappy connection!"