Post PyAV-Refactor: make sure pydub is only fed .wav or .raw files

[behackl]

If an pydub.AudioSegment is created from a file that is neither a .wav or .raw file, the library tries to interact with the ffmpeg CLI. Two possible resolutions:

• Tell users they should have ffmpeg installed if they want support for non-.wav/.raw audio files 😢
• ... or convert manually using pyav (as far as possible?)