What can I do to get rid of the prayer-related waswas that makes me hear those around me, and I fear that I am sinning?

The cure for obsessive whispers (waswas) is to ignore them and pay them no attention, so strive against them. If raising your voice and making those around you hear is due to being overwhelmed by these whispers, then we hope there is no sin upon you. However, you must endeavor to rid yourself of this by repelling the whispers, ignoring them, and not yielding to them.