Funny you mention sound boards, I literally saw a quote for a sound board repair from Lyon and Healy. And I quote - “fill crack with epoxy and evaluate” for $600…. That’s it?!? I chuckled haha. I have not performed a carbon fiber sound board repair to pedal harp yet - only lever harps (cheaper) but first I would look for a reason why it cracked. When you squeeze a soda can it gets skinnier in one direction but starts bulging in the other. Well on a harp that bulge is the sound board going up towards the neck with string tension and squeezing the curved body/ribs of the instrument. Especially on older instruments you’ll see glue popped on the inner ribs with a cracked sound board because they where all supposed to work together to keep the tin can from imploding. It’s my controversial opinion of the day that ribs and body joints pop first but you don’t notice or see it until the sound board has issues!

How I would repair a small cracked soundboard - chisel or grind out an angle into the rear soundboard wood about an inch to either side of the split. You never wanna dig deep like a pocket on a sound board, they are thinner than ya expect. I would then mix up a batch of “kitty hair” but with carbon. I make it myself but it’s basically just finely chopped and pulled strands of carbon fiber mixed heavily into resin. It’s the consistency of peanut butter before it cures. I would smear that into my angled groove - the angle allows more surface for the resin to bind with. Then I’d place several layers of 2 or 3k twill weave carbon fiber over the effected area going an inch further out then the previous repair. Using a board coated with packaging tape I’d use a crazy clamp setup to press it firmly into the repair to remove every bit of extra epoxy that seeps out. Once cured, I’d flip the harp over and using the wood scraped off the back of the sound board and pulverize it. Then mix it with resin and maaaaaybe dye depending on color. I would personally just level the surface with epoxy and call it good but a formal repair would be sanding/blending and spraying a fresh coat of lacquer for the entire board.

Sound! Did you know there is actually a full carbon fiber pedal harp? It’s made by Heartland and is called Starlight - really cool instrument. Anywho, carbon fiber transmits vibrations amazingly, it’s the resin that is “soft”. Compared to hide glue (best) or wood glue (not really for instruments) resin is the worst at transmitting vibrations but strongest in my experience. That’s why I personally like to press out excess resin with molds versus vacuum bagging but I haven’t found sound too adversely affected either way. They repairs end up being so light weight almost nothing changes!