Sound detection or recognition?

I would advise against putting the mic in the robot due to the background noises, chances are it wont provide very good results unless you are willing to spend a lot of money on a mic.

All I can suggest is look for mics for vehicle hands free, they are usually small and have pretty decent range.

I'm going to try and enclose it in sound-proof material, and then make a small opening in the head so it can hear. Do you think that would work? I'm aiming for full speech recognition.

Personally, based on my past experiences with trying to find a decent mic that I can hide but will pick up my voice anywhere in my living room, that isn't enclosed in something with moving parts... no. That's the reason I use a wireless bluetooth headset.

Does the 3/4 inch take in to account the sound proofing or is that to be fit in to the 3/4 inch gap too? I think you may be hard pushed to find something which will fit too.

It may work but there are a lot of good reasons why very few people add mics to their robots and opt for headset mics. It will also depend on what you plan to do. If it is for voice recognition then the initial instinct is not a chance it'll work at a level I'd call satisfactory. If it's for anything where dictation or free range speech is concerned such as pandora bot then you will most likely have very poor results.

@toymaker has mentioned a mic he uses that works very well however it is very expensive.

It's about 1 inch wide and 3/4 inches tall with soundproofing and room for camera, is there a microphone for that size? If it's under 100 I can afford it.

I found one that's fairly cheap. This is just sort of an idea to put into my humanoid, but do you think putting this in the backpack with the rest of his electronics is a better idea? The backpack is surrounded inside with foam and then the outer casing is a plastic.

http://www.amazon.com/Plantronics-Explorer-Bluetooth-Headset-Blue/dp/B003UV7LQW/ref=sr_1_9?s=musical-instruments&ie=UTF8&qid=1387041789&sr=1-9