Customer gives me an e-book or a document, I record audio of that document and send it to the customer. I am not redistributing that work, I only charge the customer. Will this infringe the copyright law? The customer could be giving me an e-book that he bought.
The audio book would probably be an infringing derivative work because the client could redistribute it once the client received it. It sounds very much like a product that is regularly sold by merchants relying on copyrights.
Conceivably, simply reading a book aloud to a client in some sort of streaming context that could't be shared with others or replayed would merely be fair use, much like hiring a baby sitter to read a book aloud to your children would be clearly fair use.
If there were an automatic text to sound converter as opposed to an individualized performance, it might not be considered infringing. There are people with programs that do this who haven't been sued, but the boundaries haven't been explored very thoroughly.
Honestly, there isn't a lot of guidance in this area from statutory language, and the questions would often not be guided by much case law involving similar facts. Your intuition living in the modern world is probably almost as good as a lawyer's in this situation.
So far as I can find, there is no US Supreme Court case directly on point for this issue. So called "time shifting", recording a broadcast musical or audiovisual work for later playback, has been held to be a fair use in Sony Corp. of America v. Universal City Studios, Inc., 464 U.S. 417, 421, 456 (1984). Some have urged that "space shifting" involving converting a copyrighted work from one format to another (particularly one digital format to another) should also be fair use, but there is no definitive ruling on this point. In Recording Indus. Ass’n of Am. v. Diamond Multimedia Sys., Inc., 180 F.3d 1072, 1079 (9th Cir. 1999) a court held that it was fair use to engage in “space-shifting” a copyrighted musical recording from a computer hard drive to a portable player. However in UMG Recordings, Inc. v. MP3.com, Inc. 92 F. Supp. 2d 349, 352 (S.D.N.Y. 2000) (A court ruled that medium shifting copyrighted musical recordings from purchased CDs to computer hard drive was not a fair use.
If the audio version is made only for the personal use of the owner of a lawful copy of the ebook, it is at least arguably fair use, and in any case is very unlikely to be the subject of a copyright suit. However, should the client copy and distribute the audio recording, that would pretty clearly be an infringement, and the person who created the audio version would quite possibly also be found liable.
A written statement by the client that the audio version is only for personal use, and will not be redistributed nor will further copies be made might help protect the recording artist.
This answer is quite US-centric, and the law will be different in other countries.