I believe this is the part used in the tutorial:

https://www.sparkfun.com/products/11029

There is no need for an MP3 trigger if you are using a V4 board.

if i buy a version 4 of the ezb how will i be able to add sound so my robot can talk back to me

The sounds are stored in the project that is run and stream to the EZ-B over the WiFi, which then outputs them on it's built in speaker.

i am planning my robot project.it is based on box bot but i am planning on adding grippers and an autonomous feature i also want it to have voice command and answer question that you ask it.i want to enter it for my school science fair and the judges say your project must have a use.I cannot spend a lot of money because i only have a budget of R3000 (300$) my dad is giving me the money

Be aware that if you haven't already got an EZ-B then you will not have one until June at the earliest, and that's only if you pre-ordered early. The rest of the orders will follow. So if the science fair is earlier than June you are out of luck.

Everything you listed is possible with the EZ-B.

So all I have to do is solder the speaker on the to the ezb and program it so my robot talks

How long will it take for the complete kit to come

It will possibly be available at the end of July.

The V4 EZB will already come with a small speaker on board. If you want a larger speaker you'll have to solder on a connector that you can plug a pre-amp into.