The sound for riding in a minecart is very loud by default compared to other sounds. While riding a minecart on rails, open the Music & Sound Options, drag the "Players" slider from 100% to 0%, and observe that the minecart noises get nearly silent. Drag the slider back to 100% and observe that minecart sounds are still quiet (the slider should have made it loud again); regular player sounds appear to have gone back up to full.

I suggest either making a separate slider for minecart sounds, or just making the minecart sounds quieter.