You may want to consider an amplifier. I've got an older version of this one that I use when traveling, it is fantastic:
This one however, is cheaper and is pretty well reviewed on Amazon:
I present for consideration this as another useful gadget portable smartphone BooomStick headphone amp enhancer. In-Line Wired Headphone Audio Enchancer Amp and Signal Processor right now as of this posting on sale from $9.99 to $8.39 via Amazon Prime - This used to sell for $100. Here is a review of Boomstick via YouTube. I purchased this for myself at the $10 charge when I had normal hearing because I had an old computer with a weak soundcard and not for my phone use. I keep it constantly plugged into the charger so I don't need to recharge all the time. (This is meant to be portable with your smartphone).
I found the BoomStick made my music sound great then pre-hearing loss and now that I lost hearing in one ear it helps amplify the fainter sounds. Using my home made 2E1 headphones which use Panasonic XBS bass drivers it pumps up the bass a lot. But I also used it before when I had my normal headphones in a mono-mix and it did help amplify the faint sounds.