For more than a decade, Mac users have been greeted with a particular startup chime during the booting process. With the recent release of Apple's newest MacBook Pro models, it appears the company has decided to nix this audio cue all-together.
While the reason for this hasn't been officially noted, it appears to be because of another unique change. Unlike previous models, the newer MacBook Pros automatically turn on as soon as you open the lid without the need to hit the power button.
Previously, it would wake from sleep when the lid was opened but would remain off if the computer was already in a powered down state. With this change, you can quickly and silently power on the new models, so the chime removal seems to be a justified decision.