Is prayer valid behind an Imam who hums the letter Meem in (عليهم) or Noon in (الضالين) in Al-Fatiha as if they were mushaddadah or close to it?

This subtle hidden melody in the Imam's recitation does not affect the validity of the prayer because it does not alter the meaning. Also, the idgham (assimilation) without ibdal (substitution), such as the doubling of the lam or kaf in "Malik," does not cause harm.