I'm a guitar teacher, and I do have a trained ear to reach / identify the right pitch, but my vocal tone is not pretty and is weak.

Basically, I am capable of singing in tune without any problems, but the sound of my voice is a bit weak and I can't get a stable, focused sound unless I increase the volume a lot (which is awful to practice in an apartment). Could you share any tips to help me with tone and volume control without losing grip?