You do NOT want to send only 5V to that amplifier.. At best case you will have a very low output..
A good quality aftermarket amp made for car use would never turn on.
Depending on how this amplifier actually works it will likely either not turn on or you could blow it...
Realize that most electrical devices will try to operate at a set wattage. This wattage is a combination of voltage and amperage so if you drop the voltage by half it will require atleast 2x the amperage to operate the same device. Going from 12-14 to 5 volts could cause between 2x and 3x the amperage to output the same wattage.
It doesn't make sense to provide 5 volts to this unit. Check the manual that comes with it but I believe this is setup to run directly off automotive power so there would be NO reason to use anything else if you are using this in a car. You COULD run it off the internal power supply if you have enough power left on your 12volt rail but the ratings given make me believe it is designed to run directly off the car ignition.
If you add on a sound device you want to use an external USB sound card. The reasoning is that a sound card or an onboard sound device shares the ground and other power that is within the computer. This can induce noise in an automotive environment due to ground loop issues that don't exist in home use. When you use an external USB device it isolates the power and ground of the motherboard from the audio stream which prevents any such ground loops. An internal sound card will still share the motherboard ground and power so will not help. If you use a sound card and a fiber link to an external sound processor this can accomplish the same thing. Also if you use a USB sound card all audio is sent to the card as digital data over a mostly noise immune connection. This allows you to put the USB sound card very close to the sound processor or amplifier it will be feeding and then you use a long USB cable to connect to the computer. This will prevent the audio cables from coming into contact with any RF within your car and provide a much clearer feed.
On my PC I have an onboard sound device that will allow me to run 7.1 sound or a fiber connection if I desire. I also have a couple USB sound cards I could use as well with one providing 5.1 output and the other simple stereo output.