The document notes that in some cases the phones' speaker buttons will be greyed out and service providers should first ask customers to disconnect any Bluetooth accessories connected to their phones. If that doesn't help, they should run audio diagnostics. In the case that "device could not detect dock" or "accessory not supported" alerts come up, the service provider can repair the phone. MacRumors also reports that if the affected iPhone 7 or 7 Plus is no longer under warranty, Apple says the repair providers can request an exception for this issue.
The document is apparently light on the details as to why this issue has come up. But since a repair might be warranted it seems like the software update is leading to a hardware defect. MacRumors says it verified the document -- which was distributed to Apple Authorized Service Providers this week -- with a reliable source and suggests that users escalate their cases to a senior AppleCare advisor if repair technicians say they can't fix the problem.
Last month, Apple released an update that addressed a bug causing some replacement displays to be unresponsive to touch.