Here are some guideline documents on audio preservation:

• International Association of Sound and Audiovisual Archives:
  • An Archive Approach to Oral History (1978)
  • Sound Archives: A Guide to Their Establishment and Development (1983)
  • The Safeguarding of the Audiovisual Heritage: Ethics, Principles and Preservation Strategy (PDF)
• UNESCO: Memory of the World: General Guidelines to Safeguard Documentary Heritage (Revised edition, Feb 2002), section 3 (PDF)
• Training for Audiovisual Preservation in Europe: Audio and video carriers: recording principles, storage and handling, maintenance of equipment, format and equipment obsolescence (PDF) (2008)
• Oral History Society by the British Library: After the interview (2009)
• Association for Recorded Sound Collections (US):ARSC 2015, ARSC Guide to Audio Preservation (PDF)

You can also download the following guideline documents (PDFs) written by the Archives & Records Association:

• Best Practice Guideline 6: Film and Sound Archives in non-specialist repositories
• Format Guide to Video and Sound Recording
• Selective Dictionary of Sound Recording Terms 
• Basic Technical Equipment 
• Advice on Caring for Film and Sound Recordings