The output stage of an audio amplifier will certainly take more power when the sound is louder, unless it is a Class A amplifier. The power consumption of tuners, CD players, MP3 players that don't directly drive the speakers is likely to be nearly constant for any level of sound.
LCD televisions use most of their power to provide the backlight for the screen. The power consumption does increase as the screen is brighter. One of the monitors at my work is running from a power supply with an ammeter, and the current varies with the brightness settings. That doesn't usually mean that it uses more power if the image is brighter, because the backlight is normally constant as the image changes.
Televisions with LED backlights might be able to reduce the power if the image is dark. I'm not sure about whether they do or not.
I think that CRT televisions use fairly constant power.
You can't measure the power of an appliance by measuring the input resistance, unless it is a simple heater. You need to measure the current that it takes. One of those plug-in power meters would be fine.