Babar,

If you search for electronic project articles about making a wind direction sensor with no moving parts, they'll probably explain how you need to do that.

You can think of it as being similar to how we use our eyes, two separated sensors detecting at slight offsets that can be balanced by turning the head.

There are analog circuitry tricks required. Rather than using sound magnitude balancing that requires compensation for sensor sensitivity you'll probably find that audio arrival time is probably the cheapest way to do it in analog circuitry. Audio inferometry would be too expensive for your stated design.

FOr audio arrival, if you were to use only two sensors, your robot could be programmed once getting balanced signals to do a quarter turn to determine if it was straight ahead or straight behind. Three or more audio sensors would reduce this necessity.

I've seen at least two articles in recent years about audio based wind direction sensors and I think one of those articles will answer all your questions.

-Jay C. Box