I agree with Zymmer - I'd work on making them a little less human-appearing.
Now, you could go into a background as to why their primary language is sign. For one, give them a star system with multiple stars or somesuch, so that it's never dark (drives me nuts when it's too dark to see what someone is signing)...
A very thin atmosphere, or a super-windy one, would make hearing useless as a sense, so you could make a race that is equivalent to the poor hearing disadvantage, or maybe even a race without ears at all, but other sensory organs used only for sensing presence/vibration that isn't fine enough to determine sounds, just location... (lots of tiny hairs, or something)...
Aliens without ears, or the ability to speak... sup to you - I like this a lot though, and I warn you, I'm gonna slip these aliens into my campaign too. 
Got a name for 'em?
The Doc
So you think, 'Might as well,
Dance a Tango to Hell,
at least I'll have Tangoed at all.'
-- "Rent," Jonathan Larson