A Phonos button turns blue while it plays audio, but it's not clear when it will end or what clicking it again will do (does it pause, stop, or rewind it?). If the user doesn't have audio on (muted, headphones off, etc.), it might not even be clear to them what is going on at all.
While audio is playing, one color fills up the background of a Phonos button from left to right (in LTR), indicating the progress of the runtime.
If you can see some sort of progress bar while the audio is playing, not only is it clear when it will end, but it also better communicates that it's playing audio in the first place. In addition, if the progress bar resets when the button is clicked mid-playback, it better communicates that the playback has stopped, rather than paused, so clicking it again will start the audio from the beginning instead of resuming.