Update: False alarm. It appears Samsung didn't disable it, they just changed the code.: *#0002*28345# Now instead of a '6' a the end, use '5'. Thanks, malteahrens, for the info
Looks like Samsung does not want you messing with your audio settings as they have disabled access in their latest Diagnosis app for our phones.
Late last night, the Marketplace pushed an update for Samsung's Diagnostics app bumping it to version 1122 (see above pic). While not mandatory, we took this risk to see what would happen and while most menu items are still accessible, we can confirm that the audio settings used to boost sound (covered here) are now disabled. Typing in the code results in nothing, no error, no message, just nothing.
On top of that, there seems to be no benefit to the update so if you have yet to install it, we suggest you don't.