Say for example I have hundreds of .mp3 files that I want to stretch the audio 2x in every single file, but don't want to do it individually. What software would let me do this?
You can easily do this in Audacity following the guide How To Quickly Edit Multiple Files in Audacity.
You can chain effects to be applied to multiple files. The effect you are probably looking for is ChangeSpeed. You can then select multiple files and you're good to go.