Yes ofcourse with receiver or preamp. but locking it up to max doesnt make any sense... u'll just have distortions... especially in carPC where equibment usually aint that HIFI.
Volume is controlled by a reciever, not the PC. PC volume should be locked at max always (same for CarPCs with a Head-Unit). If you really want a knob, get a Griffen Powermate and you can glue it to the front of the case :).
(BTW: I am currently building a system like this too: The first case you posted, HK TC-30, SnapStream /w PVR-500, Meedio, Girder /w USB-UIRT, 3x 200GB HDDs, DVD +/-RW DL, Logitech DiNovo Keyboard, etc., etc.)