I have a Sony Ericsson K800i phone which takes video in the 3GP format. The makers supply Quicktime to play these videos, but I'm a QT Alternative user. When I try to play a 3GP file copied from the phone, I get an error; if I cancel the error message, the video plays, but with no sound. What could be wrong?

Gadget.

Such files contain AMR audio. So you need a decoder for that. The K-Lite Codec Pack can help you play such files.