These persistent sore throats can be a real misery, especially in your case where your living depends on needing to have your voice in functioning order. If the antibiotics seemed to improve thjings, even temporarily, I'd suggest going back to see whether you can be given a stronger or different antibiotic because I believe there are "general" ones, and some which are more specific. Could the soreness be due to some other reason such as acid reflux which can sometimes aggravate the condition? In this case, liquid Gaviscon might help. Also some good anti inflammatory medication such as Ibuprofen tablets might ease the soreness.