People need to hear. Volume is only one of the controls we have for communication. Try to enunciate as clearly as you can. Speak each word so as to be understood. This may sound silly, but move your lips and open your mouth.
Speed is another control.
Don t speak too fast or too slow. Finding the balance is an art. If it helps, focus upon one or two persons you know have a hard time hearing and speak to them, so that they might hear. I am not a stickler for proper pronunciation, mainly because language itself is very fluid. It changes with time.