plagiarism and transcription of audio recordings Does it count as plagiarism if a student pays a professional transcription service to transcribe an audio recording of a social science interview/focus group for the appendix of their written assignment?

Note that the appendix is not graded as part of the assignment.
What is important here is to check the experimental protocol for any data protection/privacy issues.

Because you conducted an experiment involving human subjects, there should have been an approved protocol that tells you how the data can be handled. It is important to see if it can be sent to a third-party service, and what precautions are necessary.

In some cases, the recordings have been anonymized and nothing sensitive is discussed, so it may be OK. On the other hand, if there is personally identifiable information, or sensitive medical information is disclosed, laws, like HIPAA apply and special agreements and arrangements are necessary with the transcription service.