Pat,
The 13.8v is not related to the power output to the speaker. In this case, use the following.
Assume 4 ohm speaker.

I**2 * R = 25
I**2 = 25/4 = 6.25
I = 2.5 A

In fact, you really don’t need to compute current in this case. You know your speaker impedance. If you budget a 10% loss in the wire, you can allow a 0.4ohm speaker cable (4ohms/10). Since its wiring to a speaker and not a light or electric motor, 10% loss would be fine. You won’t hear the loss in volume. Assume 70 feet each way. 14awg wire would work fine, (2.58ohm/1000 ft).

I bought a Pocket Reference book at my last trip to the local ham radio store. Its a great reference book. Its tiny, yet has everything you could possibly think of, including wire specs. It lacks coax specs however.
http://www.amazon.com/Pocket-Ref-Thomas-J-Glover/dp/1885071000

Regards
Rich