Jtt, to get back to your question, strength of the voice depends more on having good breath control. Also, if you can relax the muscles in your mouth and tongue, and learn to lift the soft palate (roof of your mouth) you will be doing well in developing the mouth muscles. There is info in the threads I linked to (and refreshed it for you) about developing breath control. Singing technique is really hard to write about and get the info across - you'd best find a good coach who can demonstrate and then monitor your progress in developing it. It takes time and continual work to build the muscles up, just like training for anything physical. There are some classic exercises for warming up your mouth and using vowels and consonants while you sing exercises - try singing scales up and down on the sounds "mada meena mada meena mada meena mah" and "kagle ogle agle ogle ay". Lift the soft palate and relax your jaw and neck muscles. Open your mouth in a relaxed way, don't block the sound from coming out.

Alice