One possible cause of the audio symptom is if the computer either has, or thinks it has, an optical audio cable plugged into the output. Due to the way that works, it will disable the volume controls--all controls are handled by whatever is processing the audio stream at the other end, as with a DVD player or anything else that uses optical audio output.
I've read about occasional situations where the sensor within the headphone/audio out jack that detects an optical connection vs. an analog one (normal headphones, for example) gets "stuck", which would cause that symptom.
Have you plugged anything into that output recently? You might try plugging and unplugging a headphone or something similar several times to see if that causes it to kick in. There might be something you could poke with a pin, but that's just begging to break something, so I wouldn't try it.
The other possible source of that problem (other than just busted hardware, which can happen) is that the MacOS disabled that output. Open up the sound preference pane and take a look at the output section; see if the internal audio is listed and selected.
As for why the brightness, is this keyboard or display brightness? The former will be disabled if it's off. Does it still happen when it's dark in the room? What's the setting in the preferences?
