1)
Why is it written Shom'av, and we pronounce it Shom'ah?
1.
Radak: It is pronounced feminine, for it refers to the Ra'ah (evil). It is written masculine, regarding the Inyan (matter).
2)
What is the meaning of "Titzalnah"?
1.
Rashi: It is an expression of Metzaltayim (cymbals), ringing.
2.
Radak: The root is Tzalal, which has a doubled letter. This is like "l'Kol Tzalalu Sefasai" (Chabakuk 3:16), i.e. shaking due to noise.