You are on the right track. A calculation field cYearSold defined as
Year ( Sold date )
will return the year, but only if there is a date in the sold date field.
You can then add a sub summary layout part "when sorted by cYearSold" and include the cYearSold as the last field in your sort order.
This will, however create one extra instance of this sub summary part for the "on order" and "Inventory" groups on your report layout. If you only place fields in this sub summary layout part that are empty in all cases except when Sold Date has data in it, you can set all these fields in this sub summary part to "slide up", "resize enclosing part" and the sub summary part will not be visible in those groups when you preview or print your report.
I think I've done what you said. The only item I have in the sub summary part is "cYearSold". I have also set it to slide up and resize. However the cSoldDate is showing up under every record in the solds units area instead of grouping them separately.
Nevermind, I got it figured out. I needed to change my sort orders a bit more.