Does DisplayPort carry sound as HDMI does?
Yes, the DisplayPort spec is an audio/video interconnect spec. It can carry both audio and video over a single connection.
It is rare a laptop implements audio-out over a digital video interconnect. My MacBook Pros don't do it over their DisplayPort outputs. More recent MacBook models reportedly may support audio-over-DisplayPort.
I also have Lenovo Thinkpad T410. Once you have a video playing, go to Control Panel, Sounds, Manage Audio Devices.
When you're in Manage Audio Devices, you'll see a list of devices and your TV speakers should be one of the options. Just choose it and set it as your default audio device and the sound should automatically come on the TV.
From displayport.org/faq:
Q. Does DisplayPort also support audio?
A. Yes, DisplayPort supports multi-channel audio and many advanced audio features. DisplayPort to HDMI adapters also include the ability to support HDMI audio.
I wound up on this page however because my DisplayPort audio wasn't working. After searching many sites, I learned that the video driver (vs. audio driver) is responsible for the DisplayPort audio; while updating my video drivers (Intel HD Graphics 4600) I specifically noticed in the log it installed an Intel audio driver as well. After this driver update I'm able to see the TV/audio device listed under Control Panel->Sound (LG TV/Intel Display Audio), as well as under Device Manager as Intel Display Audio (in addition to my regular audio device).
As others have mentioned, it's up to the manufacturer to support/implement audio over DisplayPort. Other research has indicated that the DisplayPort to HDMI adapter must also support audio. Odds are if you're using a laptop with a common video device like Intel or AMD, they implemented audio over the DisplayPort and you just need the drivers for it.
Bottom Line: Update Video Drivers.