HELP! MP3 volume issue (NOT mix/master related..wav is fine)

[Doughead67]

Hello,

First I'll start off by saying this is NOT a gain staging issue, nor any other type of mixing or mastering issue.
My wav bounces are at the correct volume, but the mp3 version from the SAME bounce is considerably lower. It doesn't make ANY sense to me!!!
I dragged both versions into a new session and can see the waveform is actually a lot smaller on the mp3 version(peaks at about -4dB).
Several artists I engineer for have been saying their songs sound a bit lower than they used. This started after I reinstalled Windows 7, so I'm assuming the issue began after that.
I've searched and searched and only found 1 other person with this issue. Any help would be appreciated.

Thanks,
Doug
@LetoBeats