So, it may depend on the child for the music cd. My children (ds 7, ds 4) do enjoy it and listen even outside of “school.” If you grew up listening to School House Rock on Saturday mornings you’ll know how those melodies stick and can help in remembering things.
We are just going into Beta so can’t answer whether or not the completer set is needed. We have it because I purchased them from another hs’er and our 4-yr-old enjoyed building with them during his older brother’s school time (priceless).
The best advice I got for MUS was from my sister who encouraged me to invest in the wooden box (found in accessories) for keeping the blocks together. It is like a printers’ box with a plexiglas lid. This keeps the manipulatives neat and looks pretty nifty in your house as well 🙂