You cannot run voices from SD only Internal drive.
You can copy files from SD to Internal Drive by using info from this thread.
[Only registered and activated users can see links. ]

Please read the whole thread before trying, also make sure that you have the same revision Gupdate.gcd that is loaded on your device.
LDR.bin is just your boot.bin extracted from gupdate.gcd with RGNTool
The commands in update.txt will be something like this

copy,1:/English_Australian_Karen.vpm,0:/Voice/English_Australian_Karen.vpm
reboot

Where 1: is your SD Card root directory and 0:/Voice/ is the path to your internal drive voice directory