Find a role model. There are people with another problem: not able to speak loud when they are needed. But for training such people can take as an example, to observe the quiet speech. Select an influential, authoritative person, who listens, not someone downtrodden and depressed. Try to record how a person behaves in the situation of communicating with others. It can be a birthday, an event, ordinary conversations. Watch how the man has a voice.
Think of a good reason why they should speak in a particular place. Sharply to change habits, immediately switch over is nearly impossible. So choose the one place where you can practice talking quietly. Of course, there must be people who communicate with you, and they have to appear daily. It could be residential house: all the rooms say, as usual, and the kitchen start to chat in a low voice. This requires a good reason: can you imagine yourself with a scout that doesn't need to hear potential enemies. Most importantly, to the cause was inspirational, you can come up with the game.
During the week of practice to speak softly in the selected in the second step. Keep secret the process of training. Wait for the moment when others will notice that you have become something else.
Expand the sphere of influence for the second week of training. Add another place that is far from the first: if trained in the kitchen, add't room behind the wall and a café where every day you go to lunch with work colleagues. So you extend the influence of the new habits not only to another place, but on other people.
On the third week to consider the sphere of influence the whole world. Maybe quiet conversations still unfamiliar and uncomfortable. Periodically you will want to break, to quit a workout, but don't do it. In real life when this game ends, will not have constantly to speak softly. But now it is important on a physical level to accustom themselves to the new sensations: the need to train the vocal cords to get used to the reactions of others.
On the fourth week change yourself for the loudness of speech of interlocutors. Now you can talk loud and quiet. It is important to learn to balance. Observe people talking face to face or by phone. People like it when others speak with the same volume and pace of speech as they. Use this feature and get good communication skills.