Voltage can be transmitted with little to no current because electromotive force and magnetic force are directly related. This is how we can send signals through outer space and how we can use magnets in microphones and guitar pickups to transmit sound electrically. So, voltage is important for these kinds of input transducers.

Because voltage can be created with little to no current, it is preferred for sending electrical power (V*I) over long distances. High voltage in power lines can be “stepped down” to 120V to increase current to meet the demands of appliances.

However, because voltage creates an accompanying magnetic field, if we transmit our audio signals at high voltage, they will interact more strongly with nearby magnetic fields along the way (e.g., other high voltage lines or devices). That is, our message won’t make it through in perfect shape.

Resistance decreases current, and when the current drawn by a device is constant, resistance makes voltage increase (like a thumb partially blocking the end of a water hose). Impedance is basically resistance, when considering AC circuits like those carrying sound waves. So, high impedance devices like electric guitar pickups and cheap consumer microphones send their signals with higher voltages and lower current than professional audio equipment. Their higher voltage means less power is needed for amplification, but the higher voltage signal is more susceptible to being affected by external electromagnetic interference. High voltage (high impedance) may be preferred when power is the main concern, but not when the quality of the signal—the message—is important.

So, it is preferred to send audio signals at low impedance, meaning high current and low voltage. Current is manipulated throughout a signal path in order to scale the voltage as needed to preserve the integrity of the signal represented by the voltage.

Device inputs are supposed to have impedance a about ten times as high as the impedance of the signal coming into it, sacrificing current in order to boost voltage while it is inside the device. This is like “zooming in” on the signal so the audio equipment can process it with greater precision.

As for actuators, transducers that convert electricity into sound, light, heat, movement, etc., it makes sense that power is needed: measured in Watts = Voltage * Current. So, they won’t work on little-to-zero current even if we can make high voltages as described above. Every device expects to draw as certain amount of current from a circuit. If the current is constant, specified by the device, then voltage scales the resulting power. Voltage is the answer tot the question, “How much?” in terms of loudness, brightness, speed, etc.

In short, you could say that voltage is the message, and current is the muscle.