Looks like a cultural problem. Maybe they don't want to lose face by "getting it wrong". I wonder how you could adjust their criteria so that they only lose face when they don't do anything. Maybe overpraise even the most minimal attempt to participate and get student peers to join in with you, reserving mild disapproval for silence. Easy to say, I know...
Practice simple role play dialogues (Excuse me. Where's the toy department? ..... Do you have this in size 43?... ) at great length, keeping the answers minimal. (after all, in practice they're going to use the question, not the answer. Use pair work for this as then they don't have an audience. Don't worry about them speaking all at the same time - that's what happens in pubs.The background noise might in fact relieve them from feeling exposed to criticism.
If they have practiced the role plays lengthily in the classroom they should experience a little success out there in the real world on your "field trips" and this may increase their confidence.
If you have access to computers you might like to get them to work on some activities I have put online at http://mark.yates.net/engextra/ There are lots of dialogues initially done as dictations but which can then be recorded for practice by the students. They come at all levels.
They've got to speak, somehow. You learn to read by reading, to write by writing, to understand by listening and to speak - by speaking!
Good luck