Love the grammatical analysis!

'compound modifier preceding a noun...' YEAH, ROCK THAT GRAMMAR NITTY-GRITTY!

A general observation is that I see people combining more pairs of words into one, I'm sure due to influence of the internet. Log in has become login, on line is now online, etc. 

Then there's this possibility: "As the reinforcing walls of my bunker went up, a neighbor commented, 'That looks like a very sound hole you're constructing!' "  LOL 

OK, you teased the grammarian deep inside me.  Confusingly, the Wikipedia article on "sound hole" uses both "sound hole" and "soundhole," sometimes in the same sentence.

I can say for sure that "sound-hole" would be incorrect, unless the two words were functioning as a compound modifier preceding a noun, as in "He is a great sound-hole technician."

I've probably written both "soundhole" and "sound hole" without ever thinking about it.  But now that I am thinking about it, I would suggest "sound hole" is correct.  Has anyone ever written "soundboard?" "Hole" is the noun and "sound" is the adjective.  Two words.  Of course, I have no objection to the one-word version either.

This is a nice collection of sound holes Leo. With the use of laser cutting, the variety of sound holes is now endless.

Ken

"The dulcimer sings a sweet song."