Have other people told you that your voice is very soft or that you sound breathy and/or hoarse? If there has been a change in your voice you should see your doctor about whether it could be Parkinson's disease. Sometimes you might think other people are losing their hearing, when really you are speaking more softly.
What is normal? A chest cold or other virus can cause your voice to sound different, but you should go back to sounding the same when you get over your cough or cold.