Is it legal to transcribe audio without permission?

It depends on whether the audio was recorded lawfully, where the people involved are located, what the audio contains, and how you plan to use the transcript.

In many places, the bigger legal issue is recording consent rather than the act of converting speech into text. In the United States, federal law generally allows recording when at least one party consents, but some states require every party to consent for private conversations.

If you secretly record someone, receive audio that was recorded unlawfully, or publish a transcript of a private conversation, you may create legal risk. Even lawful recordings can involve confidentiality, workplace policy, school rules, medical privacy, customer data, copyright, or contract restrictions.

For personal notes from a recording you were allowed to make, transcription is usually lower risk. For meetings, interviews, classes, legal matters, or sensitive conversations, ask for consent and explain how the transcript will be used and stored.

This is general information, not legal advice.