Was the phone dropped or have liquid spilled onto it?
If not then try using the headphones with the microphone attachment and listen if you can can now hear when making or receiving a call.
Verify that you can still hear on a call when using the hands free (or loudspeaker) option.
If you still cannot hear when using the above then there is a problem with the voice reception circuits in the systemboard of the phone.
If you can hear when using the above methods then there is a problem with the phone's receiver. It may be faulty or have a loose connection.
Here is a link to the ifixit Samsung Galaxy S5 Sport Motherboard Replacement guide.
It is helpful as it shows how to open the phone so that access can be made to the receiver, to check whether the flex cable connection is OK and also to replace it if necessary.
The replacement part is available on places such as Ebay. Just search for Samsung Galaxy S5 Sport receiver part to find suppliers that suit you best.
If you find having to repair the phone yourself too daunting, then I suggest that you contact a reputable, professional mobile phone repair service and ask for a quote to repair the phone. At least you should know the cost of the replacement part.
Hopefully this is of some help
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