On one leg or two legs?

I also see one orange 12 awg. cable entering the sub panel- the black (hot ?) wire is not connected to the breaker, terminated with the cap, the white and bare ground are connected to corresponding bars. What's that about, I don't know.
01-30-09: Maril555

It appears to be a spare future branch circuit run.

Could that cause a problem?

No
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Looking at your picture of the sub panel, the neutral bar on the left side of the panel appears to have a machine screw with a round or pan head on it. The machine screw is near the top part of the bar.
Is that the case?
If that is the case, is the screw head green in color?
Does the screw look like it might be go all the way through the bar and screw, fasten, to the panel enclosure?
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In any case, I'm back to square one. What else can I do???

There is no cable, or TV on the same circuits,
01-29-09: Maril555

But do you have CATV connected to the audio system?

Did you try disconnecting the CATV company's coax cable totally from the audio/video system?

Jea48,
I will have to look at that screw when I get home, but what are you thinking about?
There is no CATV cables connected to my audio system.
The only video cable, is DVI, connecting DVD player to the front projector, and disconnecting it doesn't change anything.
As you probably noticed, the amps and preamp are balanced design, could that be a culprit?
I have a computer in the same room, but it's not connected to audio in any way, and is on a different circuit electrically.

I will have to look at that screw when I get home, but what are you thinking about?
01-30-09: Maril555

Looking again closely to the picture it looks like the screw is used to connect the left side neutral bar to the cross tie bus to the right side neutral bar..... I thought at first glance the screw was a bonding screw to bond the neutral to the enclosure. That would have been a no no for a sub panel....
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Just a note.
I think you want to put something on the other side or leg to keep the load some what even. Your breaker from the main panel to the sub pabel my not trip correctly the way you have it. Maybe you should run a 120Vac feed only not 208/230vac to the sub panal.

Hevac1,
It couldn't hurt if Maril555 moved the projector branch circuit over on the other leg, L2, of the sub panel. Might even help in balancing out his main electrical panel.
Will that solve his hum problem? No, jmho.

As for the total load that is connected to L1 leg of the sub panel, I would just about bet the total combined FLA of all the associated audio equipment is 20 amps or less.....