It is what it is. If you can find an earphone or headphone with an amplifier, or just an inline amplifier, that would work. But there's not much you can do with the phone itself unless you want to start playing in the internals (which could result in a dead phone if you don't know what you're doing) - there's a sound profile xml file you can change settings in to change carious (phone, media, etc.) sound levels in. (Look for help with that at XDA (https://forum.xda-developers.com/showpost.php?p=64194807&postcount=5, but I'd start slow - go up to 30 first and see what the effect is, then you'll have an idea of how the number relates to the sound level).