We need one pair 22AWG for one channel line level audio(Left or right).We also can use one RG6 coax cable for one channel line level audio.There is only one center conductor in side RG6 .How can that RG6 work for line level audio? It looks like we need two RG6.

For Stereo you will, You can crimp on a F(coax) fitting, and use adapter, they also make crimp on RCA Fittings for coax(Male or female). They also make dual coax.

Coaxial cable has two conductors – a single centre conductor and a metal shield (copper braid or aluminium foil) around the outside (but covered by a plastic sheath). It's called 'coaxial' because the two conductors share the same axis. With the right connectors, both conductors are used to complete the circuit. The outer conductor ('shield') is for the 'ground' side of the signal, while the inner conductor is for the 'hot' side of the signal.

Daniel

What's the application?

Existing wiring through walls? Balanced/unbalanced connections? You mention 22 AWG and RG-6 in the same sentence and I'm confused. You mean 22 AWG twisted pair/shielded cable?