Are there any job roles withing the army that would allow someone with slight hard of hearing in?

Not sure about specific job roles unless a civilian contractor you would have to be able to pass the hearing test for any job role within the Army. It all depends on the extent of it, however you can apply and the medical team will say weather you'd be a suitable candidate.

As Jordan said above, you would need to pass the hearing test (where you are sat in a little booth listening to the beebs) during the medical. If you didn't then unfortunately you would not be able to join.