Is unshielded untwisted pair cabling fine for audio?
For the connection to the speakers it's fine, for connecting audio sources to the amp's input it's not.

Both shielding and twisting's purpose is to keep noise from your signal. For shielding this will be obvious. Twisting requires some imagination. Suppose a magnetic field, for instance from a motor, may induce a current in the wires, depending on the field's direction. 



Here the currents are induced counterclockwise. In the left twist that's to the right for the red wire. But half a twist further the red wire is at the top and a counterclockwise current will go to the left, cancelling the current induced in the other half twist.  

Why doesn't a speaker's cable need the shielding or twisting? The susceptibility depends on the cable's impedance: a low impedance like a speaker (8 &Omega;) will take all the energy out of the noise, neutralizing it. A 100 nA induced noise current would give you 4.7 mV noise across a 47 k&Omega; input impedance, but that same 100 nA is less than 1 &micro;V worth in the 8 &Omega; speaker. On top of that: a speaker's signal has a much higher level than the input signal, so the noise will be relatively lower.

The most important for a speaker cable is it's low resistance. don't use thin signal wires. 



When you go shopping for speaker cable you'll see OFC cables, for Oxygen-Free Copper. The seller will tell you that it sounds better, and it will only cost you up to 100 dollar per meter(!). If you want to be swindled you definitely have to buy it. Purists will claim they hear the difference, but that is absolutely impossible. Nobody in a double-blind test could ever tell the 100 dollar OCF cable from an average 2 dollar cable. And it will never happen either.

Pick a cable with a polarity indication, so that you're sure you connect both left and right speakers with the same polarity.