VoiceMixer duck-typed the discord.AudioSource interface (is_opus, read,
cleanup) but never inherited from it. discord.py's vc.play() does an
isinstance check and rejects non-AudioSource objects, causing the voice
fx mixer to silently fail with:
"Voice mixer failed to start: source must be an AudioSource not
VoiceMixer"
Added missing `import discord` and changed the class definition from
`class VoiceMixer:` to `class VoiceMixer(discord.AudioSource):`.