Rustle's link is to Annex 1, the same site (Danish CAA which helpfully has all the ICAO docs) also has the
Supplement to Annex 1, which is where all the states' differences are filed. The US differences start on page 99. Australia also has a lot of differences.
DFC's quotes are accurate but shouldn't be interpreted as anything sensational. An international aviation lawyer should explain this properly but AFAIK:
An ICAO contracting state can comply with ICAO by either incorporating Annex 1 Standards in its national legislation or by filing differences with ICAO. The filing of differences makes it compliant, under the principle that the difference is published and available for other states to see, and judge, if they wish, whether to restrict the priviliges of another state's licensees on the basis of differences filed.
Many licenses and medicals are compliant on the basis of differences - see the document. This is mainly a technicality. One meaningful impact I think this has had in recent years was that the FAA were forced to issue a SIC Type Rating because Europe threatend to stop 2 crew aircraft flying without both pilots holding a TR.
Either way, it doesn't matter for the question IO540 raises on PPL/IR audio requirements. It is an ICAO standard that these meet Class 1 Medical level; however, ICAO compliance with Class 1 Audio does not require the Audiometry test - a practical hearing test suffices for initial and renewal Class 1 medicals, see Rustle's link page 6-7.
BTW a license complying with ICAO on the basis of filed differences is not "sub-ICAO". A "sub-ICAO" license would be one that did not comply on either basis and would have a statement about it not being ICAO compliant printed on it.