This is my wife's iPhone 4S. Mine is also a 4S. My wife's phone sounds clear and crisp. Mine has been chopping off the ends of words and calls may often be garbled. My phone is a few months beyond the warranty period, and I am not yet due for an upgrade on my phone. Fortunately, there is a fairly simple solution that made my phone's performance much better.
Step 1: Do Not Make Assumptions
For quite a long while I assumed my problems were due to the location of our home relative to the nearest towers. According to the coverage map, we are on the edge and often have only one or two bars on the signal strength indicator. I know our carrier is upgrading towers in our area and our closest tower may not be functioning, but all of that is supposed to be about finished by now.
I took my phone to a store operated by our provider. The technician there checked my phone thoroughly. Nothing seemed to be faulty on the phone. I did need a software update for the list of preferred roaming towers. Our provider also has a number I can dial to get the latest carrier-related updates.
My wife was checking on some airline miles credits for me, and was told she needed to call in from my phone number to access the account. It was then that we discovered how much different was the performance of my phone compared to her phone. My wife's experience showed us my problems are not due to the provider's tower. Could my microphone and ear speaker be defective? Plug in the earbuds with the in-line microphone and see if call quality is improved. It was improved considerably on my phone.
Step 2: Cleaning the Speakers and the Microphone
Dirt on the screens over the speakers and microphone causes problems with the audio quality of the phone, and can even cause it to go mute. See the first photo. The residue on the white end of the phone case is from my wife's facial make-up. Some of it can also be seen on the microphone and speaker screens.
A video on YouTube uses a soft toothbrush to clean the speakers and microphone. But, I had been using my phone in my garage workshop where there are iron filings from grinding everywhere, and had set it down on table surfaces. I needed a way to pull the metallic grit stuck to my iPhone microphone and speakers off of them, not just push them around with a brush.
See the second photo. Tear off a piece of masking tape about three inches long. Fold it lengthwise in thirds and press it together. Fold it over in the middle so it is now half of its length. Press the sticky folded end into the openings for the speaker and microphone. Wiggle the tape in the opening as much as possible to dislodge and grab any dirt. This worked perfectly with the iron filings on my speakers and microphone.
Step 3: What Is on the Tape?
On the folded end of the tape you can see a spot of brown dirt I removed from the microphone on my wife's phone. Fold the tape again along its length for a fresh sticky area and try again until the screens over the speakers and microphone are as clean as you can get them. Use a magnifying glass to check how well you have done.
Step 4: Do Not Forget the Ear Speaker
Also clean the dirt from the speaker that is against your ear when you are on a phone call.
In the future I will not lay my phone down in my workshop. The performance of my phone is not quite as good as my wife's phone, but it is very much improved and will work for me for quite a long time.
Update: It is possible to replace the speakers in an iPhone. There are some videos at YouTube. Some special tools are required. The speaker across the bottom is easy and little can go wrong. Replacing the ear speaker looks to be not too hard, but quite a few problems can develop during the process and it is all too easy to trash a phone. On the basis of hard experience, I recommend checking on the cost of an upgraded model phone. It probably gives superior performance for a price not much different from the cost of repairs.
But, others have found creative ways to restore speaker function after spills, or contamination with food crumbs and metallic dust. Some have removed these things by slightly moistening a soft bristle brush with rubbing alcohol. One guy puts his mouth up to the speaker screen and sucks as hard as he can repeatedly until the speaker works properly again. A guy who works in a metal shop removed fine metallic dust from behind the mesh screens over the speakers with a very strong magnet. If liquid food was spilled on the phone and dried in the mesh screen, some have poked at the holes in the mesh with a safety pin to open the mesh again.
It is possible to apply too much suction and tear the diaphragm over the speaker inside the phone. In my phone a strong magnet did not remove very fine metallic powder that had stuck to the magnet ring on the ear speaker. Also, my phone had a loose solder joint. I cannot say if I did that by dropping the phone onto carpet once, or by pulling too hard on something while working inside the phone. Bad solder joints cause garbled sound, too.