First and formost, I assume you mean the T240 HD, with the built in speakers.
Secondly, make sure the moniter is in HDMI mode, and not digital or analog mode, and is using a supported resolution.
Thirdly, unless you are using an ATI GPU with sound processing enabled, or SPDIF passthrough on a NVIDIA card, using HDMI from a GPU to the moniter will not automatically pass through sound.