From C418's Twitter, shortly deleted after posted.:
- Less annoying rain.
- New water splashing sound.
- New swimming sounds.
- Moving minecart sound.
- New, "more horrifying", skeleton sound.
- Skeleton walking sounds.
- Unique sound for big slimes.
- Spider walking sound.
- Zombie walking sound.
- Chicken walking sound.
- Cow walking sounds.
- Wolf walking sound.
- Sheep walking sound.
- Pig walking sound.
- Shearing sheep sound.
- New grass walk sound.
- New gravel walk sound.
- New sand walk sound.
- New stone walk sound.
- Ladder climbing sound.
Copied from the 1.8 discoveries & Discussion thread on Minecraft forum.