Got your note and went to look on my wife's Mint installed laptop, just installed two days ago. Using the "search" feature I have on the basement bar of the desktop, I searched "alsa" and got maybe 20 hits. I have no tech skills, but I did take a partial screen grab of one file, which might be interesting to any with tech savvy.
I put it on a temporary TLD site I have so it can be seen here.
I guess in my ignorance you will not see this plug-in except when music plays. Menu/control center/(hardware)Sound/Applications tab... and that volume slider you see there if the Applications window is empty, that is not it... You will see a different volume slider there when you play music on the browser and see the plug-in there, and can adjust it as you wish.
Wish I knew more.
After I installed Mint there two days ago, I downloaded the level 1 and level 2 updates only, and that included a later version of Firefox than the one I had on the ISO. So this might be an issue on the current Firefox version only, which is 3.5.7