[MrGoodtunes]

Pocket Player ($20) has a ReplayGain DSP plugin that can be used to adjust playback volume based on an MP3 tag with Global Gain field. But you'll have to use something like MP3gain or MediaMonkey on desktop PC to calculate replaygain and store into tag. Alternatively, you can apply the RG adjustment to all coeficients in the MP3 so any player can work. Although this is too processor intensive for PPC, there really could and should be an app for PPC to calculate RG, embed into MP3 tag, and then a free player to read this and adjust volume accordingly. Sadly, this seems to be overlooked by developers.