I swapped the keyboard for an upright piano from another mod and that worked fine, it's just the keyboard.
Now that I think about it, I had an interaction from LittleMsSam's keyboard piano sounds mod that changes the keyboard sound to piano, even though I don't have that mod. I know it's hers because it had the same gear icon as her personal objects mod, and it was actually functioning but I can't find the file anywhere. It may have left a residual file somewhere that is interfering?
Scratch that, her keyboard piano sounds mod is included in her 'personal items - instruments' mod as one file. Seems a likely cause.