the sound is made by venting. venting to atmosphere is louder than venting with the recirc adaptor but you can still hear with the recirculating setup, it just muffled a bit as teh sound is going back through the airbox.
not an impreza, but heres a clip:-
you can remove the intake snorkel on an impreza to make it a bit louder if you want. I have a standard DV on my classic which I can hear because i removed my intake snorkel so you could do that with the recirculating SSQV setup
although people install VTA DV's without a remap, unless you are mapped for a VTA DV, then you shouldnt realy VTA.
Edit:-
quick search of e-bay shows the recirc adptors for £20-40