Ok so with the Online Player, if you head into the Master Interface, you’ll have access to EVERYTHING! No need to go anywhere else, just search for the SoundSet you want. For example, if you’re running Curse of Strahd, just type that in. If you want to customise it or add other sounds in, you’ll need to Duplicate it before you can make any changes.
To create a campaign, (for example if you want to make something that combines something like an Air Elemental Battle with a Bustling Port Town) you’ll need to go back to the main website to create that campaign, decide which Offline Player you want it to show up in - Fantasy, Sci-Fi or BoardGame, add in the SoundSets you want to include and then it will show up in both your Online and Offline Player.
I hope that helps.