Making an audience isn't necessarily a matter of volume, it can be (and more often than not is) a question of tone.
A hard(ish) tone can be heard by a listener far more easily than can a broad, soft, warm one.
This can be achieved by (sometimes only a slight) tightening of the throat muscles.
MacColl's excercises, as Shimrod describes, were developed by him originally for Theatre Workshop and later adapted for The Critics Group. They are 4 basic vowel-like sounds, with two more added for fine tuning and half-a-dozen singing exercises for tonal experimentation, articulation, breath control, singing at speed and the handling of unusual intervals (large and small).
All these were designed to make actors/singers concious of how they were producing their voice and by this, learning to control it.
They take a short time to learn and master, and once learned, they stay with you for a lifetime; they are also useful in everyday life.
My family, on the male side, have a hearing problem: my grandfather, my father and his brothers, and now me - damn it!!
When my grandfather became deaf everybody shouted themselves hoarse trying to make him understand what was being said. My father found, by accident, that by tightening his vocal muscles, he could make him hear perfectly without shouting.
Incidentally, tonal lightness and slackness is one of the technical problems (among others) caused by 'head-voice' (breathinss) in some womens' singing - almost impossible to project.
Good luck.
Jim Carroll