As mentioned on the forum thread, this does sound like what I reported a long time ago, "SOUND: HUD sounds don't play when "Restrict spatialized sound to this parcel" is on in About Land > Media"". Milo Linden gave this repro:
========-
Create a HUD object with the following script

default
{
state_entry()
{
llSetText("llPlaySound",<1,1,1>,1);
}

touch_start(integer total_number)
{
llPlaySound("cf6d864b-b642-60a3-0a3c-83d15bf099ea", 1);//Soundtrack 1
}
}

Attach it to your HUD and change the parcel setting to 'Restrict spatialized sound to this parcel' Click the HUD object

Notice you wont hear any sound, although it will play if the hud object is dropped to the floor.

Workaround: use llTriggerSound
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I did some more testing of this issue. It doesn't matter what the HUD attachment point is. The things that matter are a) What region you're in, b) What parcel you're over, and c) The Y value of the HUD coordinates. In certain areas (not all areas) of SL, the HUD sound will not be heard if the HUD is on the right half of the screen (Y less than zero).

Note, the "Restrict spatialized sound" option does not seem to play a part in this. I mean, HUD sounds do not play if that checkbox is checked, but THIS issue is independent of that.

Repro steps:

Copy the above script into a plywood box and attach it to HUD center. (Might want to shrink it down a little so it doesn't block view).

Then hover on the border between Kouhun and Kitster regions, on the Kouhun side (around 142, 245, 65). Click the HUD box and the music plays. Right click box, choose Edit, slide box left and right, it always plays. No problems so far. Now cross the border into Kitster (to 138, 7, 65). Now you click the box, the music starts playing again, but if you slide it left and right, you'll only hear the music if the box is on the left side of the screen (Positive Y value).

Let me know if I left anything out of these steps. I hope you can see/hear the problem for yourself

Ultralite