Don't know if you can get it where you are, but Osha Root is my medicine of choice. I have been using it since 1989, it has never failed me.

I buy the root by the pound from Frontier Herb Company in Iowa. You can chew it, which is how the Lakota people do, or make tea from it, or use an alcohol tincture (I make the tincture usually because it is very portable. There are a few herb companies in the states who sell the tincture.

At the Sun Dance ceremonies it is common for the singers to chew it to save their voices, as they are singing all day every day for four days.
It will also knock out any kind of respiratory infection, whether in the sinuses, throat or lungs. It is antibacterial, antifungal and antiviral. I have seen it cure a very severe flu with complications when a young friend of mine who was a heavy smoker got so ill she was bedridden.In 24 hours she was up and functional, though still coughing some and bringing up gunk.

A number of times, I have given it to friends who got sick before a big performance and they were able to sing well. The secret there is to take the straight tincture on the back of the tongue and let it be absorbed as you slowly swallow it. It will numb the pain of a burning hot sore throat, and kill the pathogens. It also dries out mucous.

So it works on a stressed voice as well as a sick body.

more info here:
http://findarticles.com/p/articles/mi_g2603/is_0005/ai_2603000564/