How do you select custom weapon sounds(unique sounds with names like "wpn_rifleassaultM16A4_fire_2d") in the GECK? I can't seem to find any when i'm trying to select one(and I have A LOT of custom sounds)...

You create a sound object pointing to the sound.
In the weapon, on the art and sound tab, you then select your new sounds for the 2D and 3D sounds.

Thanks.