The title sums it all. Please help

Been' tryin' to look for that myself xD

Directly, you can't — the Scratch Team is working on fixing this.

Until then, you can actually save sprites to a file in 3.0. Put the sound that you want into a sprite, then export the sprite to a local file. Now, go to the project you want to use the sound in, then import the sprite, and extract the sound file.

Click on see inside. You have to go to sounds click on the sound you want to download then click copy then copy to new then right-click on the newly added sound in the project then click export. If the user attached the sound to a sprite. Click on see inside and select on the sprite in the bottom right corner then go to sounds and do the stuff I explained earlier,