I think my problem is the spaces in the file path. But I can't get it to work.
Here is the folder path I want to open: /Users/username/Library/Speech/Speakable Items
Assuming your path is correct it would be:
tell application "Finder"
open ("/Users/username/Library/Speech/Speakable Items" as POSIX file)
end tell
tell application "Finder" open ("/Users/username/Documents" as POSIX file) end tell - that works OK for me - I couldn't try Speakable Items as I don't seem to have that folder anywhere. I did try it with a path with spaces though and that seems to work OK too.