Meem sakinah and qalqalah: the echo rules
Quran Recitation with Tajweed · a self-paced course lesson
A meem with sukoon (مْ) has three rules, decided by the NEXT letter. Before ب: ikhfa shafawi — hide the meem lightly with ghunnah (hum), lips barely touching. Before another م: idgham mithlayn — merge into one pressed meem with ghunnah. Before all 26 other letters: izhar shafawi — say the meem clearly, no hum, and move on. Where noon sakinah had four doors, meem has three, and two of them involve the lips (shafawi means "of the lips").
The full Quran Recitation with Tajweed course · School & Courses