夏の夜の[悪夢]{ゆめ}
I don't understand why 悪夢 is read as ゆめ not あくむ.
夏の夜の[悪夢]{ゆめ}
I don't understand why 悪夢 is read as ゆめ not あくむ.
That's a technique Japanese authors use every so often: playing with furigana (rubi) glosses. You can put a different word in the gloss and in the kanji, and in this way insert two words simultaneously at the same position. The implication is something like: the title is pronounced yume "dream", but when saying "dream" they mean a nightmare.
See discussion in the comments here; and, for a deeper discussion, see Ariga, The Playful Gloss: Rubi in Japanese Literature.
悪夢 should be read as あくむ, and it means a nightmare. "夢" ipse reads ゆめ, and it means a dream. 夢 (dream / hope) and 悪夢 (nightmare) are different words.
"ゆめ" added to 悪夢 as a ふりがな (vocal notation) is 当て字, an arbitrary coinage of the author. It has no currency. You'd better not to follow / emulate it.
http://togetter.com/li/834249 「宝貝人間」と書いて何と読む?『当て字・当て読み 漢字表現辞典』のカオスっぷりが話題
当て字 is an older term.
Perhaps, the newer term 当て読み is more likely to refer to the playful or newly-invented Ateji.
http://dictionary.sanseido-publ.co.jp/dicts/ja/ateji_ateyomi/ 運命(さだめ)・時代(とき)・秋桜(コスモス)・本気(マジ)・豆富・夜露死苦など、現実に流通していながら辞書には載りにくい表記を多数採録。
Conceptually related to https://en.wikipedia.org/wiki/Portmanteau words.
かばん語(かばんご)あるいは混成語(こんせいご、英: portmanteau)とは、複数の語のそれぞれの一部を組み合わせて作られた語である。語の一部ではなく全部を組み合わせたものは合成語である。その語源からポートマントーとも。