speaker was not working.
It was mostly easy. Took me about an hour to do that. I could have done it in far less time, if I didn't strip one of the screws. I was able to do the replacement by skipping some steps.
1. The screws are very tiny. Work in a well lighted area, also use magnifier glass if possible.
2. Be careful when loosening the screws. If you strip a screw, you are screwed!
3. Keep track of each screw you open.
4. There is a tiny black triangular spacer underneath the speaker. I didn't notice it's layout, while I was taking out the speaker. I ended up leaving it out during reassembly. My phone still works fine without it. It would be nice if the instruction shows a picture of the spacer's layout.
Phillips #000 Screwdriver
iFixit Opening Tool
iPhone 4 and 4S Speaker Enclosure
iPhone 4/4S Liberation Kit