I noticed that recently the volume on my MacBook Pro has been seriously reduced, sometimes to the point of being almost inaudible. Where it's really noticeable is when I rent a movie from iTunes, then try to watch by hooking my MBP to my HDTV via mini Display Port to HDMI, and headphone jack to the input on my audio system.
Lately, even with all volume controls turned to max, it's still almost impossible to hear the movie. The answer, according to tips I found via Google, is to run Audio Hijack Pro and hijack iTunes audio. Boy, does this work. Using the gain controls available within Audio Hijack Pro, you can blast the sound with the various volume controls turned almost all the way down. The leading theory is that Apple deliberately reduced the level of audio reaching the speakers to avoid damage when booted into Windows, which apparently drives the speakers much harder.
Anyway, I can watch movies again and finally hear what's going on. The problem now is that, for some reason, setting iTunes to full-screen with Audio Hijack Pro running puts the movie on the MBP's screen, which obviously isn't what I want. The workaround I found is to go to the Displays prefpane and drag the menubar to the HDTV. Kind of annoying to have to do this, but at least it works.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20090913115726541