Believe it or not:
Vinegar is mucolytic. ("Mucolytic" = breaks up mucous.)
I know singers who gargle vinegar on stage between songs, and I know that hospitals use vinegar (well, the lab equivalent) to break up sputum samples for microbiology culturing.
And yes, I know that's way more info than you wanted, but my suggestion is to try gargling vinegar to loosen that stuff up.