Is it low even when recording videos? If it is then is usually means that one of the mics (the one on top next to the headphone-jack is dead. Had this issue on my Lumia 920, 925 & 930 (had to send all 3 to repair for a new Mic)
The microphones are dead with all applications and no change after a factory reset. Got the phone 12.24.17 so it is still on warranty. Had to buy a duplicate while this phone is getting fixed.