I had the same issue on my Galaxy S7. Sometime the output volume doesn't go well for some music files. In my case, the problem was related to those music files. The Poweramp works fine. After searching, I found the Speaker boost application. It worked very well and still I'm having this app on my Galaxy S7 device. You must try this app from google play store or follow this below tutorial, https://techshali.com/boost-speaker-volume-galaxy-s7/ I hope this information helps you and other users. Cheers!