A special mod is available for the Samsung Galaxy S6 that it boost the volume of your device. This will increase volume of both internal speaker as well as of headphone. The microphone volume is increased as well so the other party will be able to hear you more properly. This mod is said to be working on S6 Edge as well.
It increases volume while listening to music, increasing the sensitivity of the microphone, as well as in VoIP. To install this Galaxy S6 volume mod, you need to be rooted and have clock work mod recovery installed on it. After that flash the mod through the recovery.
This is said to be working on all variants of Galaxy S6.