Is it permissible to transcribe audio recordings of interviews whose owners believe no one but them will listen to, knowing that they are being recorded and will be transcribed, and that they contain personal opinions about the participants' past, present, and and future?

The default rule regarding things is permissibility, and an act is not forbidden unless there is evidence to that effect. The act mentioned in the question – which involves recording and transcribing guests' conversations with their knowledge, without mentioning their full names – presents no issue.

However, if a guest objects to their words being heard, it is not permissible to listen to their conversation, due to the Prophet's (peace be upon him) saying: "Whoever listens to the conversation of a people while they dislike it, molten lead will be poured into his ears."