2.6.1.5.1 Hearing requirements are identical for Class 1 and Class 3 Certificates. However, they are not in
compliance with the ICAO Standards for Class 1 hearing requirements.
The above is the difference filed by the US regarding the requirement for Class 1 hearing.
There is also a difference filed that US ATPL holders do not meet ICAO medical standards.
If the above is true, and is current (I can't find the URL for it, and IIRC never did find online the bit of ICAO listing the differences filed by various nations) then it means that ICAO did something which the world's biggest (by far) aviation user, the USA, regarded as pointless, ignored it, and the rest of the world has had to accept that.
It doesn't really affect anybody at present but it's bad news for any improvement in Europe, because anybody wishing to get anally retentive about this can just say they have to stick to ICAO. This is going to hold back progress in important areas, and will restrict any European IR just like the present one.