I'm assuming you've separately tried increasing the volume while the call is on the earpiece (rather than speakerphone or anything like that, since I believe those have separate controls).
Have you tried restarting the phone? Perhaps resetting network settings if not even all settings?
Sometimes some sound related settings might have effect and might be worth checking out, like noise cancellation (you can just disable and enable it again), or mono sound (also you can enable and then disable again).
Of course, could be some odd hardware thing that just popped up around the same time.