For me the word "sound" or "unsound", it is a fault dilemma. Ok, if you are going to play the Morra against Anand or Topalov possibly you are going to lose, but I believe you are gonna lose and if you play one of the "proper Orthodox" openings.
A rule I have, before I will buy an opening book, it is to see from a database games from the opening, just to play and see if I like the position for side I would like to play. Then, if I like the positions, I play some blitz games with my computer and finally I see if Carsten Hansen described this book in his column in the site of Chesscafe and he rated it with a "good mark", I combined it with reviews in Yearbooks and I buy it.
Lots of books that in the market are simply rubbish and we must be careful how we invest our money.
Closing, I have to say that Morra is fully sound, but needs a lot of studying and if I want to play an anti-Sicilian , I prefer 2.c3.

I consider the Morra to be of marginal soundness- I think White gets good practical compensation, but objectively Black should be at least equal with good play.  Probably slightly less sound than the King's Gambit (which should be equal with best play).

At the sub-2400 level, though, I think it's sound enough to be effective in the hands of an attacking player who knows the line well.  Or at fast time limits at any level of play.  At GM level in slowplay games I reckon White would often struggle to prove enough compensation.

As for GMs declining gambits, 3...d5 is popular against the Danish and Kings Gambits, though the Danish is rarely met by ...d3.  Even gambits of more marginal soundness than the Morra are often met by modest, non-critical responses at GM level, e.g. 
1 d4 d5 2 e4 dxe4 3 Nc3 Nf6 4 f3 e3 
1 d4 Nf6 2 c4 e5 3 dxe5 Ne4 4 Nc3 
1 d4 e5 2 dxe5 Nc6 3 Nf3 Qe7 4 g3
1 e4 e5 2 d4 exd4 3 Nf3 Nc6 4 Bc4 Nf6 5 0-0 Nxe4 6 Re1 d5 7 Nc3 Be6
I guess the GMs don't want to risk being caught out by opening preparation in an obscure gambit line and so prefer to outplay lower-rated opposition from positions that aren't as far "up their back yard". 

There's a book out on it called The Modern Morra Gambit that has, according to Tim Harding, revived interest in the line, and suggested that White's chances are often underestimated.

Morra is sound at U2000 but im not sure about U2200.  U2400 is definitly  pushing it. I think most GMs and IMs wouldnt think about playing it against a fel;ow Grandmaster or International Master.