When you'r listening to music, just push the + Key on the side of the phone!... Mines on the 50% position by default and it makes my ears bleed sometimes!...
Don't press and hold the + otherwise you'll skip to the next track!.
What Firmware do you have??... can't remember the sequence to find out, but iirc one of the updates killed the MP3 volume!... I'm running (I think) one of the oldest and have never had any major probs with Bluetooth (a few niggles) or MP3's...