Maybe I'm missing something or dont understand exactly what you guys are saying but...
I use PocketMusic as my MP3 player (just installed in the past couple of days) and starting tinkering with MP3's(My wife is the music nut so I bought her a Dell PocketDJ). At first I thought the MP3 volume was too loud and I couldnt control it with the PocketMusic volume slider. Then I opened X50v system volume slider and lowered the volume there also. So the app and the system volume control work together. I then also found that PocketMusic has a preamp slider on the equalizer that lowers the volume even further for pretty much total volume control. HTH.