After changing my speakers on my macbook pro 2012 mid i get a red and black digital screen and fails to boot

The connector on the Display cable and on the Logic Board are very easily damaged and it can be hard to even notice. An external display will give you the answer if this is the case. If the external display appears fine, then most likely the issue is due to the Display connector on the Logic Board if two built-in displays have the exact same type of failure. If the external display shows the same issue at the built-in display, then the Logic Board is bad. You can try disconnecting all unnecessary internal cables to the Logic Board just on the off chance one of them is causing a problem (doubtful they would cause it, but it is simple to do).

Obviously something happened during the repair. It is possible the Logic Board flexed enough during the repair that it triggered the GPU failure since the GPU failures are typically caused by bad solder joints on the GPU. I know from experience that it can be tricky getting the Logic Board back in place by having to deal with all the cables and the very specific angle the Logic Board needs to be inserted for it to fit properly.

Make sure the heatsink is properly attached and seated evenly with the screws snug. Clear off the old thermal paste and used some fresh thermal paste. I doubt the issue is with the heatsink, but that is something simple to check as well.

Which exact model? 13" or 15"? Retina or non-Retina model?

If the speakers required removing the Logic Board and I/O Boards, then most likely you have a loose cable, or you accidentally damaged something (most likely a cable or connector).

Try running the Apple Diagnostics to see if any hardware issues are detected.

Connect an external display to see whether the external display is clear. If the external display also shows the same issue as in your picture, then it may indicate loose or bad memory, or the one of the boards was damaged.