Thank you for your inquiry.
The Optical Digital Output on the LG - 55" Class (54-5/8" Diag.) - LED - 1080p - HDTV - Model # 55LB5550 is available as either PCM or AUTO. PCM is used for simple systems, Sound Bars or 2.1 systems, and AUTO is used for DOLBY 5.1 systems.
Depending on the method of connection for external audio which for this TV will either be the Optical Digital Output (Optical cable) or HMDI (HDMI cable), both of these can output either 2 channel stereo sound (PCM) or multi-channel Dolby 5.1 digital sound.
You asked if the TV will pass Dolby 5.1 through HDMI in to the TV Optical out and the answer is no it will not, those are 2 separate audio paths which you select as described above. Remember that the Optical Digital Output is a light signal. Both the Optical Digital Output and the HDMI support uncompressed DOLBY 5.1 digital audio. If your Sound Bar consists of a bar and a subwoofer, then it probably is just a 2.1 system....IFV