If turning the volume up all the way doesn't get it loud enough for you, the only way is with an external amp. (There are some phones in which you can force the internal amp to put out more power, but that comes at the expense of a few problems - overheating the phone, causing distortion, either shortening the life of the amp (which is on the motherboard and if it goes you're in for a new motherboard), killing the battery (the current drawn by an overdriven amp skyrockets) ...

Pick up a cheap powered Bluetooth speaker. They're rechargeable, they're small and they're loud.

 

There is an app called Volume+ that will bring up your speaker volume.
https://play.google.com/store/apps/details?id=com.TEST.android.lvh