Sescom just released a new cable that allows you to monitor audio on your Canon T2i/550D via a USB to 3.5mm headphone jack. The cable requires the Magic Lantern firmware hack to be installed on your T2i.
The DSLR-550D-HOCF was designed specifically for DSLR Canon T2i/550D users who employ Magic Lantern firmware in their HD DSLR video workflow. Magic Lantern on the T2i/550D enables the USB AV out port to be used for headphone monitoring during a recording, a must have feature for those serious about audio quality. This specialized cable provides an elegant and compact solution that eliminates the need for multiple adapters that are required with the standard AV out cable.