In the Sunnah prayers (except tahajjud) you should recite quietly which can be either by you simply moving your lips without sound or whereby you can hear yourself recite softly but others cannot. However, in any prayer where you are praying by yourself and you recited loudly in quiet prayer, it will not invalidate the prayer and you do not have to do any extra prostrations/sujud for the mistake (sujud al-sahu).
However, you should try your best to ignore these whispers and thoughts as best as possible and if necessary seek advice on how to deal with it.
Only Allah knows best.
Author: Shaykh Shafiur Rahman