Audio driver issue

I have one mid-2010 MP (A 5,1, 12-core) running on OS X 10.10.3 with ATI Radeon HD 5770 video card. It's connected via a mini-DP to DP cable with my Dell P2815Q (with a Dell sound bar attached, interfered through one USB port on the monitor), so I get the 4k resolution. However, I could not get audio. Though MP can recognize Dell sound bar as both an audio input (if I add a mic to the sound bar) and an output device. Was this because of the OS X? Can someone recommend a third party audio driver that I can install (hopefully an "easy installation")? I prefer not to lose the 4k resolution, so switching to a mini-DP to HDMI cable is not a desirable choice. Thanks in advance.