Hi @TiagoSestari,
You may want to check this thread.
Towards the end, there is this post by @TMosh:
"Update for other students who find this thread:
Try using “Kernel → Restart & Clear Output”, then “File → Save and Checkpoint”, then (without running any notebook cells), submit the notebook for grading.
This seems to help the grader cope with notebooks that have a lot of data in the “output” cells, like audio files and spectrogram graphics."
Good luck!