Add it to your social contract and remind people about it.
When people talk at once, most stop and it's no big deal. Some keep talking, though. Remind those folks.
I like
@ELF's trick.
I also watch who got drowned out, and next gap I ask them what they were going to say.
You can also use Turn Order or other VTT initiative.
The biggest thing is to not reward the behaviour. If people learn that the fastest person to open their mouth and makes noise gets the prize, you'll be fostering a group culture of interruption.
If you slow things down a bit and ensure everyone gets heard, people will be less worried about being first.
Example 1: The player who talks first gets the GM's ok to perform an action right away. Instead, ask what each player is doing, and then adjudicate.
Example 2: Jokes. I used to be guilty of this. Quickest reply or quip gets all the laughs. So players compete to get the fastest joke in. Running a more serious tone might help, but this is probably best dealt with a group-wide request to hold back if you think of something funny.