Enter Sony's xLOUD audio enhancement engine, which has been ported out of its primary ROM and over to rooted EVO 3Ds (and other phones). It is now a flashable enhancement (direct download here) that takes your main speaker to an 11. It doesn't affect your in-call volume unless you put it on speaker phone.
The mod appears to be universal, so there's a chance that it will work on pretty much any Android device out there, but I have only seen results on the EVO 3D. It also does not work on every ROM, so as with most things, your mileage may vary.
Now, there exists the possibility that cranking the volume up high could blow the speaker, so I recommend that you keep it at reasonable levels. And as with all root mods, please make a Nandroid backup before flashing.