Outdoor sound system speakers are subject to the elements, but some are made for just that. One of the easiest is a water resistant speaker system (the paper on the speaker drum is replaced with a water resistant material) that is wireless. A small transmitter in the house feeds its signal to the speakers on the back deck.
When splicing speaker wires it is a good idea to make an off-set joint as in the first photo.. Cut one wire longer than the other, and match the off-set on the other pair of wires. This way when you wrap the bare wires together, the two conductors don't touch each other even before putting any tape on the joint and there is much less of a lump.
If you do need to run wires outdoors, make sure the connections are protected from corrosion. Liquid electrical tape does exist, although not easy to locate.