cat-5e have 8 wires, usually 24AWG as per wikipedia.
if i'm not mistaken, using half of these (4 copper strands 24AWG) should be equivalent to 18AWG. using all of them (8 copper strands 24AWG) should yield 15AWG
using 4x runs of cat-5e (2x for POS and 2x for NEG) should be more or less equivalent to 12AWG speaker wire (at least as far as copper strand cross-sectional area is concerned)