All perfect praise be to Allaah, The Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad is His slave and Messenger.
If this helps you to remember your sins and repent of them, then there is nothing wrong with that, but you should not believe that there is any specific virtue in doing so or believe that it is a Sunnah to do so.
As regards your question about the pronunciation, then if you are asking about the correct way is to say in Athaan (call to prayer), the correct way to say is to say: Muhammad-ar-Rasoolullaah, because the "an" sound at the end of Muhammadan is merged with the "r" sound at the beginning of "Rasool" (this is called Idghaam [assimilation, i.e. joining two letters together to form one letter]).
Allaah knows best.