走漏風聲 should be a 4-character phrase, meaning the info is leaked. 例：我們正在進行一個秘密計畫，不料卻有人走漏風聲。 口風不緊 is not a 4 character phrase, in my opinion. It can be divided into 口風 & 不緊。 口風不緊 means that one tends to spread the news that he/she was told. In contrast, if one is good at keeping secrets, you can say「口風緊」，如：那個人口風很緊，你可以放心跟她說心事。

As you can see, they both denote the same meaning but the nuances are different as in English. 走漏风音 = cat out of the bag 口风不紧 = loose lips

走是走后门的走（to feel this verb） 漏是漏水的漏（Ditto） 风声是 the air from your mouth 口风 is the same

I have never heard 走漏风音, I think it is the same as 走漏风声. 口风is a word which means the information revealed in one's words. If someone is described as "口风紧", it means that this person can keep secret. Examples: 这件事我一直没有告诉过任何人，怕走漏风声。 他口风不紧，和他聊上几回便能套出许多我想要的消息。 I think they are usually used in writing.

Do you mean you want to know if they are idioms or not? IF it is, the word 走漏风音 is exactly an idiom, and the word 口风不紧 isn't. maybe the reason why you can find 走漏风音 in the dictionary is you have searched the wrong words, you can try it again with word 走漏风声 .About 走漏风音, it comes from a classical chinese phrase named ‘隋唐演义’ ，you can have a look at this book if you are interested in it. You can consider it as a whole thing which can be a predicate, an attribute or an object. As for 口风不紧, it is just a word short for never keep ones lips buttoned.