Add type hints to `audio.py`

[Dr-Blank]

Description

• mypy passes for audio.py

related #1296

Type of change

Please delete options that are not relevant.

Checklist:

• [ ] merge #1302
• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my own code
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have added or updated the docstring for new or existing methods
• [x] I have added tests when applicable