Another thing worth mentioning is if the disc was recorded on a standalone audio recorder, as opposed to being burned in a computer's CD writer, then it has to be a kind of CD-R that's specifically for audio.

Despite the fact that these discs are marketed as "optimized for audio", they are exactly the same as normal "data" CD-Rs, but just include a piece of data saying they are audio CD-Rs. (And they are more expensive because of a RIAA tax!) Standalone recorders won't record onto CD-Rs that don't have this flag.