Here is my experience and solution for this problem. I just purchased a Sony Multi Channel AV Receiver (STR-DG729) to control all my video/audio devices.
I connected a FIOS cable box, Panasonic DVD/DVR, and Canon HDV Video Camera with HDMI inputs. I connected the SONY HDMI out to HDMI in on a new DELL 24-inch LCD monitor.
I connected a SONY DVCAM video deck to the sony with an RCA analog L-R audio cable and to the DELL with an S-Video cable.
I connected a pair of Moonson computer speakers and subwoofer (previously connected to my MacBookPro) to the SONY receiver.
Finally, I connected my MacBookPro to the SONY using a high quality Belken mini to L-R RCA audio cable from the headphone jack out and to the DELL with a DVI cable from the external monitor port.
All components worked fine--except for the MacBookPro when the external DELL flat screen monitor was connected with DVI. When I switched the SONY to monitor the MacBookPro audio, I was greeted with a underlying rumble/hum. At first, I tried to ignore it, but it was just too annoying. Of course, lowering the volume with the SONY removed the hum, but then I couldn't hear any audio from the MBP. After consultation with SONY, Apple Support, Best Buy (where I purchased the SONY) and Radio Shack, I was convinced the problem was a 60 MHz hum introduced through the house electrical system.
WRONG. After unplugging the MPB, the hum persisted. I then decided to start disconnecting peripherals from the MBP one at a time. Out came the FW800 PCI Express Card, the FW400 cable to the DVCAM deck, the Belken USB Hub with several items attached, and finally, the DVI cable to the DELL monitor. THAT STOPPED THE HUM. Without the DVI connection, sound from the MBP through the SONY was perfect.
Next, I turned off the DELL and touched the DVI connector to the MBP. THE HUM RETURNED. I swapped out the DVI cable for another one. The hum persisted. I touched a DVI cable to the MBP from an adjacent NEC MultiSync Flat Screen monitor connected to my G4. THE HUM RETURNED. At least that meant the new DELL wasn't the culprit. IT WAS THE DVI connection.
Just to be sure, I swapped out the Belken mini-to-RCA cable to the SONY. The hum returned as long as I had the DVI cable connected.
Earlier, I had spent about an hour on the phone with SONY tech support double checking all the connections on the receiver. Now I called Apple Support and spoke with a level II tech. He was very patient and reviewed the entire system. He made an appointment for me with my local Apple Store to check out the MBP. He also suggested that I connect audio to the SONY with either a USB cable or a TosLink optical cable.
At the Apple store, the Genius was stumped. I suggested he search this forum and he came up with this thread. At the store, I purchased a XtremeMAC XtremeHD 4-meter TOSLINK optical cable with 3.5 mm optical digital audio jack connector for the MBP (TN408LL/A at $29.95).
When I returned to the office, I re-connected all components and installed the TOSLINK from the MPB to the OPTICAL TV IN on the SONY. Moment of Truth: I booted up the MBP then turned on the SONY and selected TV. No hum. I played a video over the Internet. Great sound, no background hum.
I did discover that the TosLink optical cable disables audio control on the MBP. Says so right in audio preferences. Audio level must be controlled via the SONY while the TosLink cable is connected.
So there is a Day-plus of my life trouble-shooting the hum. This is definitely an Apple MBP bug. The hum is generated by the DVI conenctor and passes through to the headphone jack out. You won't hear it with computer speakers connected directly to the headphone jack. But when you connect to a high-powered receiver, it amplifies the hum.
I've been detailed in this description to save others from my frustration and loss of so much work time. I'm contacting Apple to see if they acknowledge this bug. Hopefully they will and will roll out a firmware/hardware fix before my Applecare contract expires.
