Attendance at hearing tests
One thing to be clear on, employees cannot refuse to attend a hearing test provided by their employer as part of their duties under the Noise Regs, as long as the test is taking place in the employees' normal working hours. It is not optional and the regs even specify that - in regulation itself rather than merely in guidance. This is in Regulation 9 of the Noise Regs. If someone refuses then treat it within the disciplinary procedure as any other non-compliance.
For employees who are found to be at risk, follow-up action may be necessary, for example by talking to their line manager about possibly swapping them to a quieter job, or providing different stronger hearing protection, or that time off may be needed for follow up visits to a specialist to examine any issues further. In all these cases the person responsible for the audiometry programme should sit down with the employee and get their consent for that. I've never come across people refuse that when it is explained properly as it is for their own benefit.