Try dishwashing detergent.
Sweat is produced in apocrine sweat glands and it is the sweat from apocrine glands that contains proteins and fatty acids which make it thicker and give it a milkier or yellowish color. This is why underarm stains in clothing appear yellowish.
Most dish washing detergent has the ability of remove oils and fat from surfaces. So rub a little concentrate on the stains and leave for awhile. Soak in warm water then pop into the wash. You should be able to remove most of the stains unless they've already been set in.
How's that for an answer?