You might want to check out this clip from the game Darksiders. It's basically a boss fight with a giant worm thing and might give you some inspiration (you have to skip to about 2.20 to get to the relevant bit):
I guess because worms don't actually do much screaming and shouting you have a bit of a carte blanche to make the vocalisation sound however you want. The above clip isn't anything revolutionary in terms of monster sounds but it's sonically solid and the earthy rumbling is definitely something to consider. Are worms blind? I don't know but maybe it could be more of a psychic/echolocation thing like a bat?!
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You might wanna think about adding some sounds to emphasise the danger element of the worm. Worms don't generally produce a sense of fear when confronted with them (although I'm sure a giant monster one might!), but something like a snake which is kind of similar in shape has a definite fear factor associated!
Maybe trying something like a hiss or rattle type sound could help with that. I remember in the new War of The Worlds film those tentacle/probe things had a sound like a rattle snake which helped add the sense of danger about them.
Perhaps layering in those types of sounds with some heavy whoosh sounds could be useful for the attacking element. Not sure how your worm in question moves about but something like a punch bag being dragged along a rubble/gravel type surface and pitched down might help add some more body and menace to it.