Students can get confused when they use multiple play-sound blocks in a row because if you don’t use the play-sound- until-done blocks the sounds start one after another before the previous sound can finish playing. Students can also get confused when broadcasting messages because there are broadcast blocks and broadcast-and- wait blocks. I recommend students use "until done" or "and wait" so that their blocks of code execute sequentially.
Example: "Remember to use the broadcast and wait block."
See this and related tips as a Tip Sheet: http://csteachingtips.org/tips-for-teaching-scratch