Hi jeet,
Given the proportion of surround content vs. the main left and right and center channels, you should be OK with the AWG18 gauge wire. Using thinner than recommended wire simply results in increased resistance, in ohms, which means you'll be "wasting" some amplifier power because your receiver's surround amplifier has to "push" the AC audio signals harder to overcome the increased resistance of the thin wire.
You certainly won't hear any difference, nor will there be an degradation of the audio signal's frequency response over the audible bandwidth.
The wire won't heat up significantly because of the thinner gauge.
Regards,
Alan
