Key to controlling the volume of your singing voice is controlling your breathing. Beneath your lungs is a muscle called the diaphragm. When you inhale deeply, the diaphragm lowers as the lungs fill, shifting the position of the stomach and small intestines. The diaphragm helps to control the amount of air that is exhaled when singing by supporting the lungs, which is important for helping to improve breath control, and by association volume control, when singing. Proper breathing technique along with practice will enable you to maintain quieter or louder notes for longer with more consistency.
Place a hand on your stomach and breathe in -- your belly should be the first thing to expand and your shoulders should not rise. If you are unsure or don’t feel you are breathing correctly, lie flat on the floor with your hands at your waist and take a deep breath. Fill your belly evenly over a count of five seconds and exhale over the same count. Repeat ten times and remember how it feels to breathe correctly.
Stand up and breathe in deeply over a count of five seconds and exhale over the same count. Concentrate on inhaling and exhaling evenly so that you do not feel like you are running out of breath or letting go of too much air too early on. Repeat this exercise ten times to form one practice. Try to practice once a day.
Pick a comfortable note to sing and hold. Breathe in over a five second count and sing that note while breathing out over the same count. Try and keep the pitch, tone and volume of the note constant. A good note to pick is one that would fall into your regular speaking register – too low and the throat is too relaxed, too high and the throat can become strained. Repeat this exercise five times, altering the volume of the note each time by starting very quietly and increasing up to a mid-volume then returning to very quiet over the length of the breath. Choose a different pitch for every breath.
Repeat the exercise in Step 3 using all of the different vowel sounds - a, e, i, o and u. Each vowel sound requires the mouth to move into a different shape, altering how much air you may let out of the duration of the exercise, so it is important to practice with different sounds.
Practice these exercises on a daily basis and you should soon see a marked improvement in being able to maintain notes consistently at various chosen volumes.
Once you have mastered these exercises, increase the time that you have to hold the breath/note/volume.