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Short time voice loss could mean that she has an upper respiratory infection. Viral illnesses are also a possibility. If this is the case, your cat should recover with time. Keep an eye on the secretions from the nose or eyes. If they change color, the cat may need some antibiotics.
Long term voice loss could be a result of more serious factors and you should see the vet to rule out an obstruction, cancer, etc.
I am leaning toward the short term causes because you are seeing coughing as well. If this continues, you will want to see your vet to get an exam and possibly antibiotics.
Let me know if you have more questions
Since she seems to be struggling, I would see the vet first thing tomorrow. There are no over the counter meds that are safe for cats.
Neither green nor yellow secretions are normal. If you see this, she has an infection and you need antibiotics.
If she's having trouble breathing through her nose, you can use a little pediatric nasal saline drops (not adult) in each nostril to clear the secretions.
I would not change her diet at all because this can cause severe gastric upset.
If the secretions from the eyes and nose are clear and she's eating well, you can wait this out and see if it improves.
If she stops eating, you are in big trouble and you need to see the vet.
I had another thought about this. I would also be concerned about FIV/FelV since she is an indoor outdoor cat. This can sometimes contribute to the overall wellbeing of a cat when these diseases are present.
When you are able, you will want to see the vet to have her tested.
I also wanted to suggest that your cat might benefit from having a humidifier in the room or boiling a pot of water on the stove to add some humidity to the air. This can help to clear the secretions in the nose and throat and make her breathe easier.
Some cats will refuse to eat when they are stuffy. If this is the case, warming the food will sometimes help them be able to smell it and then they will be more likely to eat.
I hope that this helps more!