In the US, if one legally records a conversation with a celebrity, is one then allowed to use snippets of the recording (say, too short and generic for words spoken to be copyrighted) to produce a song without the celebrity's permission? Auto-tune songs are often examples of these kinds of songs. If it is legal, is one allowed to credit the song partially to the celebrity?
A legal recording could be with consent or without, depending on the state you are in. If the person speaking is speaking publically, consent is irrelevant; if they are having a private conversation with you, then it depends on the state. In about 20% of states including California, all parties have to consent, otherwise just one party has to consent. Assume this happens in California, which has the most restrictive rules. So for the recording to be legal, the person must be informed that they are being recorded (if they continue to talk having been informed, that is implied consent). That is just about legality of recording.
Additionally, there is a property right (in California), "personality rights", which limits ability to exploit another persons image etc. California Civil Code section 3344 prohibits using name, the voice, signature, photograph, or likeness of another, either in a product or in advertising (but not "news" type purposes), without their prior consent, and violating that law could lead to a hefty damages award. Consent to record is not the same as consent to exploit, so if you ask Tom Cruise in a private conversation "can I record you?", that does not constitute consent to exploit his personality.
There is an open question as to whether a First Amendment argument can overcome. In Nussenzweig v DiCorcia, a case in New York, the suit was over an analogous photographic exploitation. In that case, there was a debate over whether the use of the photograph constituted "art" or "trade", and the photographee lost on First Amendment grounds. In this case, a person's photograph was used (without consent) in an art collection which was sold, but the court held that the right to artistic expression trumps the requirement for consent. However, the court also recognized that states differ in art standards, citing Comedy II Publications, Inc. v. Gary Saderup, Inc., 25 Cal 4th 387, a case in California. The crucial difference between New York and California is that in California, art is "transformative" and not just "duplicative" likeness. The court formulates
a balancing test between the First Amendment and the right of publicity based on whether the work in question adds significant creative elements so as to be transformed into something more than a mere celebrity likeness or imitation
and states that
What the right of publicity holder possesses is not a right of censorship, but a right to prevent others from misappropriating the economic value generated by the celebrity's fame through the merchandising of the "name, voice, signature, photograph or likeness" of the celebrity.
So, potentially there is a First Amendment defense of such a use.
"Crediting" the speaker might be seen as a false endorsement of a product, and one could run afoul of the Lanham Act, 15 USC 1051. The issue would be whether one could reasonably infer that the subject had in some form endorsed or given consent to such a usage, and a statement like "We thank Tom Cruise for his voice snippets" could easily give such an impression. A statement like "Snippets of Tom Cruise's voice were obtained by recording at a public rally in Winnemucca" is less likely to imply endorsement; if there are hundreds of such credit-attributions in the work, the impression of consented endorsement is lessened and the impression of "harvesting" is increased.