The volume controls must be set before you play the file for Free Hosts. For Premium, you can adjust file volumes while you are live on air, while the file is playing.
If you set the default to 100% when you upload it, then 100% will be the default on your switchboard. General rule of thumb is that if you want to talk over the file, you will want to use 25-50%. If you want to play the file outright with no voiceover, you should set the file at 100-150%. If you set the volume much over 150% there is a good chance you will hear sound distortion in the podcast