Be sure to set your webgl build to play in browser for your final submission.
A new soundtrack will be significant if you are making the music yourself. If you are changing the sound theme using music from elsewhere be sure to credit it and describe your choices in your final submission description so it can be potentially counted as a modification. If you are creating the new background / scenery your self that will certainly count as a significant modification, if you are using assets from elsewhere be sure to give credit and to describe your choices in your submission description so it can count as a modification.