The Equalizer is one thing I use on almost every audiobook I listen to (listening on Android's "Smart Audiobook Player" -- which, as an aside, is by far the best player on Android if you're looking for general feature/usability inspiration) It just has a super basic 5-frequency equalizer that you can save your setting as a general preference in 1 book, and then load into any other book later on. I usually increase the lows and decrease the highs because the balance of audiobooks makes