I went from Hypo to hyper with no apparent cause/reason. So get your TSH checked to make sure and work with your endo from there to see if its a medication problem or a new problem.
I was diagnosed with Grave's Disease in Nov 05 (hyperthyroid) and had radioactive iodine treatment. I'm not a doctor but this sure sounds like it could be related to your medication. I too, have always been a singer, and found my thyroid disease because I was getting hoarse and lost some of my lower range. I had a check-up and was then diagnosed with Graves. The ultrasound showed that I had a lot of nodules on my thyroid, which is what could be causing your hoarseness. The thyroid is located right around the vocal chords. I would make an appointment to see your endo doctor right away so they can re-evaluate your thyroid, look for nodules, and adjust any medication. One thing that I have learned over the past two years is that you have to be very proactive with your thyroid disease, know your body well, and report any changes to your doctor, even if it means multiple visits. Your thyroid controls a lot more in your body than any of us can imagine and it is very important that things are regulated property. Hyperthyroidism, if you could be experiencing it, can cause heart problems. Armour Thyroid medication is full of T3 hormone that can affect the heart. Good luck to you and please make an appointment with your doc.
Since you are taking Armour, you should probably have a Free T3 run every three months as well.
It is possible to go from hypo to hyper for many reasons.
Over-medication is one. Thyroiditis is another.
That's why you need to have a TSH and a Free T4 run every three months.