I too have the Acer Aspire 4750G, however I use Linux Mint 11, though it is based on Ubuntu 11.04. I too also can't adjust brightness, however I was able to fix the plug-in earphones to work, or the headphone jack. The way I was able to fix it was that in the BIOS, I changed the graphics card from switchable to integrated, and all of a sudden the headphone jack started working. But if you want to play games in Windows, you have to switch the graphics card from integrated to switchable. I'm not sure why changing the graphics card settings affected the headphone jack. If sound doesn't work when you changed it, go to sound preferences and in hardware make sure Analogue Stereo Duplex is selected and in output select Analogue Speakers.
I'm not sure why changing graphic card settings fixed my headphone jack, but it might be because the Intel chip controls the headphone jack and when you use switchable, and since nVidia Optimus is not supported officially in Linux, that some aspects of the Intel chip like sound cannot be used. Hope this fixes it!.