FreeAdvice® Legal states that:

Copyright protection is available for original works of authorship fixed in any tangible medium of expression.

We interpret this as follows:

If you own the original content of the audio or video (i.e., the participants have transferred the rights to their contributions to you, and you wrote the interview questions), then you may be able to copyright your transcript.

If you haven’t conducted the interviews or created any of the content (such as the questions), then the work would generally not be eligible for (your) copyright.

But it’s best to get advice from an attorney well-versed in copyright law and describe your unique case.