There was a topic that asked for this same feature, and we found out that that's not a very good solution. When the stop all sounds block is ran, the block still waits until it would be the end of the sound. Here's my solution
if you don't want to copy the script, you can also just go here to get the block Snap! Build Your Own Blocks
btw, here's the original topic that asked for this as well, add missing sound block