I've found that the difference between minimum brightness and full brightness is about 5W of power consumption (15.4" screen).
Wifi uses maybe 1-1.5W. Onboard speakers shouldn't use that much -- maybe 1-2W, even cranked up.
My Athlon 64 machine uses around 20-24 W (for the entire system, as reported by /proc/acpi/battery/BAT0/info) with lowered brightness and wifi.
I'm curious exactly how much less power the Turion uses than the Athlon 64. I have a suspicion that it's less than we might think. Most of the increased power efficiency of the Turion over the Ath64 is at high P-states; my Ath64 uses gobs of power at full frequency, but gets 3 hours on an 8-cell charge at 800 MHz.
The advantage of Turion, I suspect, seems to be not battery life (since it's not that much more efficient at low frequency) but portability, since a lower maximum possible power output at full frequency means a less bulky cooling design. My machine's pretty thick (1.75"), just because when the processor is cranked up, you've got to dispose of all that heat somehow.
(That said, it really doesn't run that hot. I worked on a coworker's Inspy 700m yesterday, and the thing was much warmer than mine.)