This is a pretty small thing, but is there a way for the volume sliders/widgets to stay synchronized when they are changed in another mixer? So as I change volume in alsamixer for example the volume slider/icon would move to reflect that.

One other little thing, currently there is the "close to tray" option, would you consider adding a minimize to tray option?

I guess you ar eusing mpg321/... or mplayer (with v1.0, v1.1.x use the software volume option of mplayer)
There are some commented-out lines that check the volume regularly. I don't know if there is a better way to do that. I must say I haven't looked much, I prefer using software volume.

About the volume when using v1.1.x with gstreamer and pulseaudio, gmusicbrowser currently use the option (if available) to use pulseaudio the gmusicbrowser-specific volume. It has a few shortcomings, in particular changing the gmusicbrowser-specific volume in pulseaudio is not reflected in gmusicbrowser volume icon/slider. Fixing this currently has stability problems (probably the same that makes the gapless option unstable, ie : a threading bug with the glib perl bindings)

About the "minimize to tray" option, I'll think about it. The only problem is that I don't want to add hundred of options (I know there is already a lot).