I have this problem to some degree. When it is a normal environment not so much, but say when there is significant background noise like in the town centre, near traffic etc. and moreso in louder places such as bars and especially if I have been drinking (any fluid).
I think it is a combination of my voice naturally not being very loud and obviously producing slightly too much saliva.
So the solution is therefore to somehow increase my vocal power and reduce the excess saliva. Is there anything I can try apart from swallowing every time I want to speak?