Why can I not hear when people talk to me unless using my speaker

Sounds like you have a most likely have a damaged ear speaker. If you have recently had a screen assembly changed it might be the front camera flex cable was damaged or improperly installed.

It should be a pretty simple fix but if you had the device repaired recently I would take it back and have the person who did the repair look at it. If it worked before a repair then they should repair it for free and if not I would find a different repair shop in the future.