Google Chrome	67.0.3363.0 (Official Build) canary (64-bit)
Firmware Version	Google_Samus.6300.276.0

Intermittent, happened twice over ~2 hour exploratory testing session

Steps to repro:
# Launch YouTube, play video of choice that contains sound. Note: I was using YouTube in an Incognito window but I'm not sure that detail is relevant to this bug 
# Start Select To Speak 
Expected: speaking continues while music/sound keeps playing
Actual: sometimes as soon as STS start speaking, the music stops. Even after STS is done speaking, sound would not resume. Reloading the video would resume sound. 

ChromeVox consistently speaks over other sounds. 
Note: this has nothing to do with the concept of "audio ducking", it's simply that all background sound is stopped unexpectedly.