problem with dolby digital driver

Welcome to the Dell Community  @pratik pratik 

To better assist you, we would need the exact model number of your laptop/desktop???

U2

PLEASE DO NOT POST ANY SERVICE TAGS!!!

Windows 8.1 supports Dolby Digital with appropriate drivers, but unless your system has a SPDIF output, then Dolby Digital wouldn't be handled by the Realtek driver.  If you're trying to send Dolby Digital through HDMI, then you would be using the HDMI audio driver that would be included with your GPU driver, likely Intel or NVIDIA depending on the system you're using.  Note that Dolby Digital Live is a completely separate thing.  DD Live involves on the fly encoding of mutli-channel PCM audio into a Dolby Digital signal.  That can be useful for things like games that are generating dynamic audio rather than having a pre-recorded Dolby Digital track, because "native" multi-channel PCM audio can't be carried over SPDIF, but Dolby Digital 5.1 can be, so if you only have a SPDIF connection, Dolby Digital Live allows you to transmit multi-channel audio even from games.  But if you just want Dolby Digital output from sources that actually have a Dolby Digital audio track, such as movies, you should be fine.