The BIOS initializes this at startup, so really nothing has a chance to write to anything before it's enabled.
The first 3 register writes by the BIOS are:
Code:
$4022: $00 ; disable IRQ if active
$4023: $00 ; diable I/O?
$4023: $83 ; enable I/O, immediately after
I have no idea if any games turn it off later, though. I wouldn't presume it's necessary to emulate for compatibility, but I am not sure why you'd want to leave it out either.
Since the topic refers to it as "audio" enable, if you're asking if an NSF player needs to implement it, there are no FDS NSFs that rely on it to my knowledge, and they all presume it has been enabled before the NSF code is run. (I think most NSF players do not implement it, but again, not sure why you'd intentionally leave it out.)