What is C2?
On all CD-ROM media are at least two levels of error correction, called C1 and C2. If both fail, the output is probably not correct anymore. Most drives are not able to report if audio reads failed or not, so each block had to be read twice and be compared to make sure that everything is fine. But some newer drives are able to report if C1/C2 failed on specific samples on a read, making it possible to read only once and see if a read error occured. But there is still a problem, as some drives do not report these errors correctly, so you should test it thoroughly before trusting the results.