Heat is the graveyard of energy. Everything that uses energy, or is energy, is actually just energy on it's way to the graveyard.

The energy of the universe is a pool of water a top a cliff. Water running off this cliff is used to do stuff (work), and the pool at the bottom is heat.

The "heat death of the universe" is referring to this water fall running dry, and all the energy being in this useless pool of "heat".

Almost none. A long time ago a friend and I did the math for sound, photons (status LEDs), etc and it was a rounding error of 1% or something silly like that.

And that’s ignoring that sound and photon emissions typically hit a wall or other physical surface and get converted back to heat.

It all ends up as heat in the end, just depends on where that heat is dumped and if you need to cool it or not. Most watts end up being even more than the theoretical heat per watt due to said cooling needs.

There is literally no way around the fact that every watt you burn for compute ends up as a watt of waste heat. The only factor you can control is how many units of compute you can achieve with that same watt.

All sound will end up as heat.