What is the sound effect for "frown"? (How do you pronounce the kanji?)

The basic onomatopoeia is 'musu' (ムス, often seen as むすっ or むすっと) here written 'mussuu' (ムッスー) for added emphasis.

mpgrey said:
What is the sound effect for "frown"? (How do you pronounce the kanji?)

Useful food for thought, kanji is only those characters which are typically more complex and come originally from chinese. Onomatopoeia words, grammar ending, colloquial words, and foreign words are usually written in Kana, which has two of its own synonymous denominations. Hiragana, Katakana.

I stand corrected. Thanks for telling.