Let's look at some information from an "Electrical Engineering Training Series":http://www.tpub.com/neets/book10/41c.htm
"LENGTH OF A TRANSMISSION LINE. A transmission line is considered to be electrically short when its physical length is short compared to a quarter-wavelength (1/4 l) of the energy it is to carry. [and a few paragraphs down] When power is applied to a very short transmission line, practically all of it reaches the load at the output end of the line. This very short transmission line is usually considered to have practically no electrical properties of its own, except for a small amount of resistance."
Now, one quarter-wavelength of an electromagnetic wave at 20 kHz (the highest audio frequency) = 0.25 * (speed of light) / 20 kHz = 3750 meters = 12,300 feet = 2.3 miles.
Therefore, if your cable length is short compared to 2.3 miles, then up to 20 kHz it "has practically no electrical properties of its own, except for a small amount of resistance", so you don't need to worry about the 75 ohm characteristic impedance of the yellow cable or the 50 ohm of the red and white cables. Characteristic impedance
is a different electrical property from DC resistance, which is the only property of concern for a "short transmission line" (all your cables, when used to carry analog audio signals).
On the other hand, analog video signals lie much higher in frequency than audio. At those high video frequencies, the same (physical length) cable no longer behaves like a "short transmission line". So it is recommended to use a 75 ohm (characteristic impedance) cable for video, to match impedance of the connections.