Can I ask you if she is showing any other abnormal symptoms involving her upper respiratory system. Is she sneezing, wheezing or having eye or nasal discharge?
Is she eating and drinking normally?
How long has this benn going on? How old is she? Is she up to date on her vaccines?
Does she go outside?
Thanks and I will await your reply.
Ok, now I have a better picture of things.
Most of the time, the reason their voice changes is inflammation of the larynx or voicebox. The most common cause is secondary to a viral infection. Anything that could cause irritation to the back of the throat or upper airways could be responsible and that could include any mass, polyp or even inhaled foreign body like a blade of grass. In older cats it seems that sometimes we see this voice change and there is no virus involved. It may be related to fibrosis or scarring of the vocal cords due to aging.
If it doesn't resolve or gets worse, or if she develops any other respiratory signs, it would be a good idea to schedule a thorough exam for her with your vet. It may require sedations to get a good look at her larynx and pharynx to fully evaluate. There may not be anything that can be done, but it is always good to look.
I hope this helps and please let me know if you have any further questions.