I just spent an hour on Google trying to find:
a) Any Portable Audio Players that use Replay Gain metadata to level volume
b) Determine what kind of replay gain tag these players recognize

All I've been able to track down is the fact that replay gain support is in development for the Neuros (most likely for MP3 and possibly vorbis), and that iTunes or MusicMatch in combination with the iPod have some sort of volume leveling solution (though it's not clear to me whether the iPod reads some sort of tag and adjusts the volume accordingly, or if the iTunes/MusicMatch software actually converts the audio track to a normalized volume level before transferring it to the iPod). Other than these products, it seems that most portable players don't support this (critical?) feature.

Does anybody have any more information about this? To provide a bit more information, I'm working on Songs-DB, an all-in-one media jukebox that currently analyzes volume based on Replay Gain, but doesn't yet embed this information within track tags. We want to embed this metadata within track tags, but aren't clear about the best way of doing so for MP3.

Rusty

p.s. a beta of Songs-DB was just posted today to:
Songs-DB Beta Forum - Beta 5

If you use MP3Gain on your MP3's, the file itself is altered. The hardware will play back the file with the gain adjustment.

I know that it's possible to to actually modify the volume levels of all the music in a collection. What I'm really trying to find out is 'for people that don't want to physically alter their tracks do any players out their support a means of adjusting volume using a precalculated volume adjustment coefficient that is embedded within the MP3 tag?'.

In addition, I'm curious as to what the combination of the iPod and iTunes actually do (does iTunes actually modify the tracks? or is a volume coeeficient used in some way, and how).

I only keep a casual eye on the portable market, but I'm not aware of any that support Replaygain. I think it would have received a major mention on this site if one were released.

Bear in mind that Replaygain isn't even natively supported in any of the major and popular player software for computers (such as Windows Media Player, Quicktime, Real player). An exception is certain format plugins for Winamp, such as Ogg Vorbis, or the Replaygain DSP. Foobar2000 is strictly for enthusiasts, and the Notepad-a-like GUI probably deliberately puts off the "kewl" and "l33t" masses who want funky looks above functionality and quality of audio.

What's popular at HA isn't necessarily popular enough in the world at large for reactive (rather than pro-active) manufacturers of custom ICs and firmware developers to add such simple features. I doubt that even Ogg Vorbis or MPC support will actually have ReplayGain functionality built in at first (and quite possibly won't even do gapless playback).

So MP3gain, and the MP2 lossless-gain equivalent via BeSweet seem to be the only options for portable users at the moment aside from applying a ReplayGain scaling at the time of encoding. (FB2K's encoder plugins including the flexible foo_clienc can do this, and John is working on a Lame which calls Wavgain first to analyse the files and calculate the --scale factor)

I'm fairly certain it doesn't modify the track itself as you can switch "sound check" on and off both on the iPod and in iTunes and hear an instantaneous change. Further support for this is the fact that if you look at the mp3's track info it tells you how many dB + or - the track is.

There doesn't seem to be much public info available on the mechanism though.

I am not aware of any.

'Volume Leveling' (as it is known in MusicMatch) is already being promoted as a significant feature in MusicMatch (which is a very popular player, especially in the OEM market (bundled with a variety of portable players).

Because this is a real need, I think it's only a matter of time before this type of feature makes its way into mainstream portable players. One of the main stumbling blocks is probably the lack of a clear standard (in terms of tag format).