Google+ hangout mic volume on iPhone4/the new iPad
Unfortunately, there are no options for adjusting Mic volume in iOS, as it's handled automatically by the audio subsystem. I tested this on my iPhone and where was no problem with the volume at all. If you have a case on your iOS device, make sure it's not one of the many designs that inevitably muffle the Mic by being so close to it. Secondly, I would suggest trying a pair of earbuds or Apple EarPods, anything with an inline Mic that works with your iOS device, and try a hangout then. If the others are able to hear you clearly with the earbuds/EarPods w/ inline Mic, but not so clearly when it's just the built-in Mic, I would consider taking it to Apple and at least have them look at it. It's quite possible your device could have a somewhat faulty Mic. I've had a few iPhones with faulty parts that were manufacturing errors, and Apple replaced them free.

Sorry I can't offer more help, but there's no options for adjusting the volume of the Mic in iOS, so I would look at eliminating possible physical matters.

Good luck and happy hangin'!