Audio Problems With Lubuntu 13.10 Installed On MacBook

[Aere Greenway]

All:

I installed Lubuntu 13.10 (64-bit for Mac) on a MacBook today, and it 
seems to be working well except for one problem with the audio.

If I use the laptop speakers, all is well.  The volume controls are as I 
would expect, and the volume level is good.

But when I plug in the audio cable plug (that connects to my stereo 
amplifier) into the headphone plug on the Mac, I get no sound.

On investigating it more, I discovered that if I (in pavucontrol) slide 
the volume slider all the way to the right, I can hear a faint sound 
through the amplifier.

I also noticed in pavucontrol, that when I plug in the plug to the 
amplifier into the headphone plug, the device switches from "speakers" 
to "headphones", and the speaker output is muted.  I think this is 
correct behavior.

I tried looking at the audio controls in more detail in QasMixer, but 
didn't find anything that would fix the problem.  If I set it to "Line" 
output, and use the "Line Boost", I get no output.  It seems I must use 
the "headphones" device.

Does anyone have any idea why the headphone output is so terribly faint?

-- 
Sincerely,
Aere