Tens of thousands of pages【名词短语】 

        on which 【which引导定语从句】

        (secretly recorded定语) conversations 【定语从句主语】

          are transcribed【定语从句谓语】

从以上分析可以看出，这句话中心部分是名词短语Tens of thousands of pages，后面是定语从句对其进行修饰。也就是说，本句话只是个句子片段，不是完整的一句话。

Tens of thousands of pages on which secretly recorded conversations are transcribed.

参考译文：成千上万页的秘密谈话录音/记录被记录/抄录/转录下来。

【1】on which secretly recorded conversations are transcribed.为定语从句修饰前面名词“tens of thousands of pages”关系代词which指代tens of thousands of pages,并在定语从句作介词on的宾语，on which为介词短语在定语从句作地点状语，修饰过去分词recorded.

【2】on which secretly recorded为过去分词短语作定语修饰名词 conversations

【3】are transcribed为定语从句谓语部分

备注：on which secretly recorded conversations相当于conversations which are secretly recorded on tens of thousands of pages

这是一个完整的句子：Tens of thousands of pages是主语， on which secretly recorded conversations 是过去分词短语作主语的定语， on which 在过去分词短语中作地点状语， are transcribed 是谓语动词（被动语态）。