My bottom speaker was blown in my 4s, I suspect it was caused by humidity in the bathroom while showering.
The repair went pretty smooth. I ordered the plastic opening tools, replacement speaker, and liberation kit for my phone. I got the two pentalobe screws out no problem. Replacing the speaker was pretty easy. The only parts that worried me was peeling up the dock connector cable (which came up very easily) and peeling up the battery (which I decided to just leave in like other uses have stated doing). Peeling the antenna off the speaker had me worried too, but by working it slowly I was able to get it off without breaking it. The only real problem I ran into was in my Liberation kit, it came with one phillips and one pentalobe screw. I emailed iFixit and they promptly sent me a new liberation kit with two phillips screws. Great customer service on their part!
My advice is to not take out the battery, however you still do need to disconnect it from the logic board to disconnect the cellular antenna. The battery is really glued into the phone and I felt like I was going to either destroy the battery or my plastic opening tool.
Another tip, when you slide the back glass up, you need to use a bit of force to make sure when it is slid up, it comes apart from the phone easily. The first time I slid it up very gently and it wasn't enough to come away from the phone.
Be sure to keep track of what screw goes where, I like using little magnetic dishes to make sure I don't lose any. And if you're having trouble getting a screw to back where it needs to go, just run the tip of your screwdriver over a magnet a few times, that way your screw should stick the the screwdriver.
My final bit of advice is to take your time and read steps twice.