Sorry to ask what might be an obvious question but can you upload an mp3 file into the online player to be able to play speech to other players in your online group. I have some pre recorded files that i want to be able to use without modification and wondered how easy it is to do in the online player
Yup. I’ve even done it live during play sometimes… ie suddenly thought… “you know what would be a really cool song for this moment”, quickly made an element, uploaded a file and hit play. 
For a good start with everything you need to know about creating content in the Online Player… see here: