Hi there - I see it's 5 years since my last post on the jaw...
Since the jaw is such a large muscle, re-forming it, in the best sense of the word, is going to take time, so exercises for it should be done little and often (may take several months.) The more powerful muscles, which we use for chewing, will give way to the finer muscles which will allow freer, less effortful movement, which is what we need for vocal health.
1) Side-to-side: Learn to move your jaw sideways; start by going all the way to the left and right (no need to stretch, here) - use your fingers either side of your jaw to help. Now try to make the direction-changes very smooth. (Can your fingers move the jaw on their own?)
2) Up-and-down: Same procedure - try to make the direction changes smooth; the jaw will want to "snap" up from its lower position, so try to eliminate that;
3) Round-and-round: combine the two, make big circles (in both directions). Smooth movement. Aim more and more for effortless movement.
4) Smaller circles with jaw open. This is the final exercise, and the one I find most useful. We do need to sing with our mouth relatively open (1 to 2 fingers-worth), and we need to be able to move it easily in this open position, and to be able to control the vocal chords in this position (e.g. the arytenoids). But this is not a normal position for us, so these things are not easy. Finally you can add sounds, and breathing in noiselessly, whilst making the small circles. I found it affected my speaking voice quite distinctly, and my habitual method of producing the consonants would cause a slight jaw lock-up (big muscles jumping in), so these are the things that need a lot of time to re-form, to re-learn.
Anyone who just says "relax the jaw" without appreciating the difficulties in doing so is not helping you greatly.
Happy singing, happy vocal explorations!
