vgm_vol: I think the main problem is the lack of documentation about vgm_vol. The readme of the tool collection is so outdated that it even lacks a section about vgm_vol.
Replay Gain: According to Maxim's in_vgm source, the old Replay Gain feature made use of an SQLite database to store all the gain values and sorry, but I'm not going to add something that large to in_vgm. (The file for the SQLite implementation has about 2 MB, that's about the size of the entire VGMPlay source code including sound emulation.)
Also, Replay Gain won't help at all if the VGMs suffer from clipping.
VGMfreak:
1) The YM2612 has 4 "operators" for each channel, each generates a sine wave. Usually, you set one frequency for a channel and each operator gets that frequency multiplied with a value from 0 to 15. Operators can be turned off and on individually.
You can combine the operators in various ways, e.g. making one operator affect the sound of another one. You can also add them, so that they behave like multiple seperate sine waves played at the same time (and play e.g. the notes 1+5).
The special mode of CH3 just allows you to individually control the frequency of each operator, but the amount of playing notes is not different to other channels.
Technically you can play up to 6*4=24 sine waves at the same time on the YM2612, but the frequencies and sound characteristics will be very limited.
To make it short, CH3 is not more or less polyphonic than every other channel, but you have a bit more control over the used frequencies.
I think some songs that use the PMD driver made a very clever use of the YM2203's CH3 mode to play 2 or 3 notes on CH3.
2) There were many variations of sound cards with OPL2 chips. I heard that some of them didn't use Yamaha's original FM chip, but custom ones that sound more or less similar. Also the output seems to be filtered differently, depending on manufacturer and model.
I like that strong bass sounds of my SB16, but the heights are a bit more filtered than on my YMF744.
3) By "emulators" you mean something like Winamp plugins that do similar things as in_vgm, right?
In this case, I know that there is
in_fmpmd and I once had a plugin that plays MDX files, too.
You maybe should have a look at
Modland. They have about everything related to module and game music. (FTP links are at the very bottom of the page.)