These are screenshots of a grouped product page. The first shows all products
in the group having the box checked  for
"Enable this to only allow one of this item to be bought in a single order."
This limits the quantity to one.
[ct_spacer height="10"]As soon as the one tangible item is unchecked, the counter appears and the
add to cart button is moved below the table. It's not obvious that the button
at the bottom of the table is only for the CD.
[ct_spacer height="10"]If all are unchecked, all of the add to cart buttons disappear but the one at the
bottom of the table. This would make sense if they were all tangible items, but
who is going to order multiples of the same MP3?