Any expenses you receive from Action on Hearing Loss will not affect your benefits, as they are a refund of payments you have had to make.
So, volunteering will not affect your Incapacity Benefit, Disability Living Allowance, Income Support or Job Seekers Allowance. If you receive Job Seekers Allowance, you have to actively look for paid work as well as volunteering, but the Job Centre should give you a bit more notice if they call you for an interview.