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As you said, you have chronic post-nasal drip with a change of voice. According to me, it is chronic sinusitis with chronic laryngopharyngitis.
The diagnosis of chronic sinusitis must be confirmed and I suggest you get a CT PNS (paranasal sinus) done and send me the pictures.
For now, I suggest a Fluticasone nasal spray twice daily, tablet Montair LC (Montelukast and Levocetirizine) once daily and capsule Pantoprazole once daily.
Also, take steam inhalation and avoid high pitch singing while you have a sore throat.
One more thing, I want to know is that whether your voice has become hoarse or you cannot sing because you have a feeling of a dry throat.
For further information consult an ENT otolaryngologist online -->https://www.icliniq.com/ask-a-doctor-online/ENT-Otolaryngologist