This ability gives the person who has it the power to completely and accurately mimic the voice of another person. The more time spend listening to a voice, or sample, the more accurate the mimicry will be.
Learning a Voice
The more samples and time spent with a particular voice the better. The higher quality the samples, the better the mimicry will be. In reality, the best source to mimic a voice from is the actual person themselves. Spending time listening to and talking to the actual target of the mimic will result in the best and highest quality voice mimicry.
Samples of high quality audio can also be used, the more words, tones, inflections, and such that can be heard over the recordings allows for a better mimicry of words and phrases that fall outside of what has been directly heard. Vocabulary and mood are important!
Limits
The power does not give the person the ability to memorize a voice, for that one either has to have eidetic memory or the Perfect Memorization ability as well.
This ability also does not give the person the power to know if such a person has favored word choices or phrases they use often in different settings. Research is required to dig these out, though if exposed to enough variety of sound samples or being near the target during enough different settings, the gaps can be filled.