Startup chime audible despite headphones?

At startup, my Intel iMac always sounds the startup chime through the internal speakers, even if I have headphones plugged in. Naturally, depending on what game I was last playing, the chime can be quite loud, and is irritating when I restart while others are sleeping. This is the first Mac I've had that the startup chime was not automatically sent through the headphones if they were plugged in. Is this a "feature" or a glitch?

One side point, the headphones are stereo (two color bars), and are standard issue from Sony, however the stereo plug does not fully insert into the headphone jack. About 1/16th inch of metal rod remains visible when the headphones are completely plugged in. These are the same headphones that plug in perfectly to my former G4's front headphone jack. I was guessing the iMac's headphone jack was shortened to accommodate other styles of plugs (perhaps optical?). The headphones operate correctly, and during normal operation all sound goes through the headphones, but at startup the chime still sounds through the internal speakers.

I'd like the startup chime to go away, and am hoping it's not a hardware issue or a bad jack. I don't know if it makes a difference but the external speakers jack is just as "short" inside as the headphones jack.