Sounds nice. :)
The de is hardly audible in the example. Very quick. I guess that happens sometimes in real life too...
alles is muziek!
are singing should also be an acceptable translation
The children and the birds are singing is an accepted translation, and has been for at least 2 years.
I skipped the "de" before vogels, and got an error. Was this because it is a listening exercise, or is it verboden to leave out the second article in Dutch?