If it is a Toshiba, then there is a pair of speaker cables that you can pull out to disable the speakers. Of course it would depend on the model. You can find some guides on how to take a Toshiba laptop apart here.
I just replaced the motherboard in my Toshiba 2455-S305 myself, following part of the guide here. As you can see on “step 15” there are 2 speakers cables that need to be disconnected (The lower pair in the photo). It’s pretty simple to get there, and since you don’t replace anything it should be straight forward. The only tricky step is when you remove the “securing trip” (easy to break) pay close attention to the orientation of the metal piece that the middle screw is holding (in step 9 of the guide above).
I took plenty of photos when I replaced the motherboard so if you have a similar model to mine, you’re covered.