How to keep sound from muting while using dictation?
You need to set some hidden preferences to make this work (OS X 10.9 - Mavericks). Open Terminal and enter the two commands below:

defaults write com.apple.SpeechRecognitionCore AllowAudioDucking -bool NO
defaults write com.apple.speech.recognition.AppleSpeechRecognition.prefs DictationIMAllowAudioDucking -bool NO


After doing this turn off dictation in Systems Preferences, wait a few seconds and then re-enable it. You should now be able to dictate while audio is playing. I’ve only tried this while using a headset/headphones, it’s probably not advisable without. :)

To restore your system to it’s virginal state, run these commands in Terminal and then restart dictation:

defaults delete com.apple.SpeechRecognitionCore AllowAudioDucking
defaults delete com.apple.speech.recognition.AppleSpeechRecognition.prefs DictationIMAllowAudioDucking


Found via: https://gist.github.com/rmangino/8388849