I had the same issue where my call speaker didn't make any noise, I had to use headphones or speakerphone. It was my ribbon cable - only one side of it was severed, resulting in no call speaker, plus my activity/notification LED did not work. Replacing the flex cable may do the trick, but if you're ordering and replacing that, you may as well order and replace the call speaker too.
PS: my ribbon got torn more after that, which resulted in my display not working, but touch input was still working