Remixing Audio for Smartphone Playback

Looking for any advice or links to tutorials on how to optimize audio playback on smartphones.

Audio tracks on my videos sound good when playback is over traditional speakers. However, on smartphones often background or music beds are not heard mixed with voice, etc. I am guessing that sounds/music/voice need to be kept in closer db range to optimize playback on smartphones.

With most web content being viewed on smartphones, I was hoping to find a tutorial or best practices for optimizing audio playback on those devices while maintaining clarity on traditional audio systems.

Any help/advice/direction would be greatly appreciated!

Thanks!~