MJL  I'm using the regular internal speakers.

Here are some more details: if I start NoiseUB with headphones plugged in, the program produces noise. That was also the case with the original PPC version of the program. If I then unplug the headphones, I get a spluttering sort of noise from the MacBook's speakers. If I then click Silence and Noise again, I get pretty consistent noise through the internal speakers, but it's interrupted slightly by typing, by opening programs, by opening tabs in Firefox -- in short, by any activity. There are small gaps that seem related to any other activity. NoiseUB isn't using huge amounts of CPU or memory -- 2.3% CPU, 14.2 MB RAM. If I quit NoiseUB and open it again without headphones plugged in, there's no sound. Hope this info helps, and thanks for your work on this helpful program.  
(Version 1.1)

MJL  Here's the error number:

1852797029  
(Version 1.1)

MJL  Oops -- as you may suspect, I'm relatively new to the Mac. I'll get this number when I'm at my MacBook later today and post it here.  
(Version 1.1)

MJL  Thanks for your reply! Here's what I found in the Console:

2007-10-23 20:15:38.864 NoiseUB[402] NoiseGenerator::startAudio

2007-10-23 20:15:38.970 NoiseUB[402] NoiseGenerator::AudioDeviceSetProperty setting buffer size status

2007-10-23 20:15:40.313 NoiseUB[402] NoiseGenerator::AudioDeviceStart status

2007-10-23 20:15:46.017 NoiseUB[402] NoiseGenerator::AudioDeviceStop status

2007-10-23 20:15:48.177 NoiseUB[402]

There are several more instances of starting and stopping, but no indications of an error. There's a ten-digit number that follows the setting buffersize and the starts and stops, the same number each time. In case it's a number in some way specific to my Mac, I've omitted it here.  
(Version 1.1)

MJL  I was really excited to see this program as a Universal Binary. But it doesn't work on my MacBook -- no sound. (On my MacBook Pro, it's great.) So for now I have to stick with Ocean Waves for pink noise.  
(Version 1.1)