Don't blame me if you break your broken speaker cones lol.
This wont properly work on metal or plastic cones.
This fix will likely not make your speakers sound better than they did before they cracked. It should however sound better than they did when they were cracked.
Step 1: Required Tools
Elmer's glue, gorilla glue (this stuff also works great for fixing cracks in speaker dust covers), or another type of semi flexible wood glue
A regular coffee filter.
A paint brush. I recommend a semi stiff bristled brush as it helps to rub the glue into the speaker cone
and last but not least a broken speaker...Duh
Step 2: Mixing the glue.
Your first step is to water down the glue so it will properly soak into the speaker cone. Mix the glue with one part water to three parts glue. I wouldn't recommend adding any more water as it could cause the glue not to stick or make the speaker cone all soggy.