iCanHazShortcut - Simple shortcut manager for OS X 10.8 or higher.
Mutify - Use a shortcut on any Mac device to mute your microphone
CustomShortcuts - Customize your Mac menu keyboard shortcuts for free.
Smart Tab Mute - Ensures that only one tab is playing sound at the same time
Keysmith - Create custom keyboard shortcuts for your Mac and the web
Spotify Ad Blocker - Mutes Spotify (and only Spotify) during ads. Portable, with a minimal UI via tray icon.