I haven't really noticed Sound Forge doing anything to the quality as
long as you go MP3 in to MP3 out only changing the volume. I
understand if it has to uncompress and recompress the file, you will
be losing stuff.

if you don't hear any issues then I would't worry about it....but
there are programs that can change the level of an MP3 without uncompressing and recompressing.

MP3gain is one example.

http://mp3gain.sourceforge.net/

use caution when downloading any exe from the internet.

I have standardized my mp3 at a level of 89. When I get an mp3 that is really loud, I run it through mp3gain set for a level of 89.

mark