How can you set a standard volume level for all slides?
Hi Betsy. I got this information from another discussion stating that there is no option to update the volume level across multiple slides as each audio file exists independent of the others. You could instead try to change the publish settings similar to the below. Hope this helps. Thanks.
You might also consider tweaking the volume of your individual audio files so once they're in Storyline they all play at the same volume. Once you've accomplished that task, the one volume knob in Storyline should be sufficient to control your audio. I've used the free program MP3Gain in the past to give all my files the same volume level. There are tutorials on YouTube to help you figure out the software.
If you have sound effects that are supposed to be louder/softer than your spoken audio, then you'll have to deal with those individually.