I have a video where I deleted the animation part and I only have the sound.
When I try to save the file it saves as
.blend, how can I save it as an mp3 file. I have already set the start up file to save as mp3.
In Properties > Output, set something like this:
It's not ideal, but maybe it'll work for you..
Output format to .mp4 then use an online converter to change it to .mp3. I use http://media.io/