Having somewhat intermittant loss of sounds. Is it fixable?

My iPhone 4S has lost it's sounds (keyboard, videos, Pandora) but sometimes the keyboard will briefly sound. It does ring and I do hear texts but I think sometimes it doesn't as I miss calls when I should have heard them ring. The volume buttons don't always work/display. Once in a while the screen goes black but then comes right back on where I was. The speed seems sluggish. I took it to the Apple store who looked at the moisture indicator which was red and said that was my problem. I have never had this wet except for some very little water by that I mean more than a few drops but nothing that touched the entire phone or depth. It was hardly anything. They tested the battery and said it was bad. This is a replacement phone sent me several years back and the battery has never been better than it is now so not sure if she just wanted to get me to buy a new phone but obviously didn't want to run diagnostics. What is the prognosis. Anything that can be done? Maybe it's a coincidence but I noticed it after we left our charger at a hotel (had the cord) so bought one at the airport. However, my husband's phone is fine.