How do I add audio to my animations/artwork in Adobe Fresco?

I don't believe you can use Fresco to add audio.

You could try Adobe Express to create a Video, I think that should allow you to add the animation as well as Audio.

Alternatively Adobe Rush, that will definitely allow you to combine the files.

Lastly you could head over to Fresco UserVoice, which you can access via App Settings > Help > Suggest a feature and ask for audio to be added to Fresco.