Image of standard 4-conductor speaker wire.
This is industry standard, 4-conductor speaker wire. It is typically wired with red/black as the main positive/negative at each speaker. Some speakers may support bi-wiring, or bi-amping, so it is worth pulling at least one in-wall speaker off the wall to see exactly how they wired it up.
Red = + (right speaker, or if only one speaker, this is just positive)
Black = - (right speaker, or if only one speaker, this is just negative)
White = + (left speaker, or if only one speaker, this is often not connected, or used as positive for bi-wiring)
Green = - (left speaker, or if only one speaker, this is often not connected, or used as negative for bi-wiring)
The colors and how they are connected is established industry standard. Unfortunately, not every installer actually follows this industry standard.