If you want to use one cable and get video/sound it has to be display port. You can get a minidp>dp adapter to use your MBP and then a DP cable to connect to the monitor. Make sense? I also have a 2560x1440 monitor and use a minidp to dp adapter then dp into the montor.
Wow, I didn't think a displayport carried audio. That definitely seems like the best solution. Thanks!