What exactly is the diamond mark printed in some mushafs in Surah Hud, verse 41, under the ra in "majrahaa"? Does this change the pronunciation?
This marks the only place in the Hafs reading of the Quran where imala is applied.
In the Hafs reading, you don't pronounce the vowel in the middle of "majraha" as a regular aliph. Instead, you pronounce it like the letter "A" in "fate". You should find an audio sample of the ayah by a good qari to get it exactly right.
This is called Imala, and the word in the Hafs reading that has Imala is the Ayah in your question. It changes the"Fathah" a A sound to a "Kasra" a "e" or "i" sound, and it changes the "Alif" to a " Ya' ", so instead of saying "Majraha" you say "Majreha" or "Majriha".
You can hear it from a good reciter at Qur'an Explorer, chose Abdul Basit as the reciter or the down from him.