When using the pliability app on your mobile device you have the following options for audio:
Voice w/softer music
Regular combination of voice and music
Music w/ softer narration/voice
For example: As a way to lower the music volume, you can tap on the dot between "Voice only" and "Original". That will lower the music and make the voice louder. And to lower the overall volume, you can still use your device's volume buttons.
You can access these settings by tapping the sound icon and sliding your finger left/right across the audio settings bar:
Desktop Web Browser
Controlling voice and music separately is not yet available via web browser.