>>171
There are rules about 促音 (sokuon) in the said on-yomi combinations. But the rules stem from old Chinese pronunciations which are no longer used in modern Japanese or Chinese, so they are very complicated and not practical.
If you want an in-a-nutshell explanation, it may be helpful to know that it often occurs when the first on-yomi kanji ends with a "k" or "t" series and the second on-yomi kanji begins with k, s, t, or h (f).
In your example, 作 ends with "ku," which belongs to k series, and 曲 begins with "kyo," hence it satisfies the above condition.
達成 (tassei) is another example of this kind. 達 (tatsu) ends in t series while 成 (sei) starts with s.
There are some exceptions to this rough rule. For example, 的確 is often pronounced "tekikaku" ("tekkaku" is also acceptable though).