Head rush
OK, had the same problem with the internal speaker. Just stopped working one day after being in my pocket while coaching baseball practice. At first, I thought it was because I had been using the headphone jack in the car and (having had the same problem with a Palm Pre 2) I figured the headphone jack was in la la land.
Tried the blowing and I tried cleaning the jack with a q-tip and a dab of alcohol. Nada. What had me thinking was that if you adjusted the volume, it showed the speaker icon. If you have a headphone plugged in and adjusted the volume, it would show the speaker icon with "headphones" above it. That led me to believe it wasn't an erroneous headphone jack problem. I did try removing/inserting the jack 20 times and it didn't work.
I left the phone in the car holder the next day for a few hours while at a site and decided to drive to the Apple Store which was down the road. Right before I got there I tested the internal speaker and it was now working. I figured the sight of the Apple Store scared the iPhone into submission.
Well, went to baseball practice again a few days later, phone in pocket and guess what? Yup, stopped working again. Didn't work after a few days so I was concerned. Started reading more posts about this. Tried the "reset all" and that didn't work. Thought about an errant app but really hadn't installed anything around the time it stopped working. Then thought about it again logically. What was the same both times it stopped working? Baseball and long, legged training pants (it's been cold) with semi-fuzzy pockets!
I took the protective cover off and gave it 5 really, really hard blows into the headphone jack and, voila, it started working again. Seriously, I tried this the first time but they were wimpy blows. This time it was head rush, dizzying blows. I hope this helps someone. Now I am going to look around the house for an old phone that has the headphone jack plug and insert it into the iPhone.