Questions 12-13 are based on the following.

Record companies defend their substitution of laser-read compact discs (CD’s) for the much less expensive traditional long-playing vinyl records in their catalogs by claiming that the audio market is ruled by consumer demand for ever-improved sound reproduction rather than by record manufacturers’ profit-motivated marketing decisions. But this claim cannot be true, because if it were true, then digital audiotape, which produces even better sound than CD’s, would be commercially available from these same record companies, but it is not.

疑惑之处：What on earth does record companies defend?  substitution of laser-read compact discs or laser-read compact discs?  

我觉得substitution of A的意思就是 A的替代品；但是从下文看来，这个substitution似乎指得就是CD

请xdjm们帮忙翻译一下第一句话吧。谢啦

这里需要指出楼主的一个理解上的错误 substitution of A for B的意思是 用A代替B

substitution of A并不是A的代替品 而应该理解为把A作为替代品

第一句话的意思是：

唱片公司在为其使用 镭射压缩光碟 代替 价格较为低廉的传统的慢转黑胶唱片作辩护时指出 音像市场是由用户对 更为逼真的声音复制 需求所决定的 而不是由 唱片公司的 利润导向的市场决策来决定的。 

翻译的不太好，LS将就看吧

Record companies defend their substitution of laser-read compact discs (CD’s) for the much less expensive traditional long-playing vinyl records in their catalogs by claiming that the audio market is ruled by consumer demand for ever-improved sound reproduction rather than by record manufacturers’ profit-motivated marketing decisions.

个人觉得,你的理解是对的.

唱片公司坚持其换代措施--在产品目录上,以CD来取代以前便宜的传统vinyl唱片,因为市场对更高音质的音乐制品的需求胜于对唱片制造商对市场决策的利润驱动(即市场要的是质,但公司想要的是钱).

你从第2句话也可以确定,第1句是这个意思.因为第2句就从这句引申到Digital Autotape来说事的.

个人理解,希望没误导你.

另,参考substituting 用法:

“substituting moral power for physical force”(Elizabeth Cady Stanton)
“用道德力量来取代物质力量”(伊丽莎白·卡迪·斯坦顿)