This was pretty difficult to figure out; The eSpeak documentation isn't all that good.
espeak -ven-us+f4 -s170 "6 new wireless networks found"
-v option to specifiy a voice. After that you can provide the type of language, such as en or en-us. After that, add a plus, then either
f, and a
s option lets you set the speed. Unfortunately, most of the voices sound pretty bad…