Connecting the audio between the cable box and the TV is easy, if you are using one of the composite or component video inputs. Just plug in the associated R/L audio jacks in a line with the video input. If you are using the HDMI input, that carries sound on the HDMI cable. You don't need to connect the audio out of the cable box to the TC-P50C2. See the connections page of the TV manual here: https://www.manualslib.com/manual/246619/Panasonic-Viera-Tc-P50c2.html?page=12#manual
. Then set the TV to this input.
However, the Panasonic TC-P50C2 does not have an analog audio output. You could try connecting the cable box RCA audio out to the Bose Wave system. You will need a RCA M/M to 3.5 mm adapter cable. (Alternatively a RCA M/M to 3.5 mm F converter and an AUX (3.5mm M/M cable or a 3.5mm M to RCA F/F adapter with a standard RCA audio cable will work between the cable box and the Bose ) This may not have the audio sync correctly with the video.
The alternative is to use a digital to analog audio converter and one of the above RCA to 3.5 mm solutions. Get an converter that has an optical digital audio input and an optical audio cable (Toslink). (The cable isn't always sold with the converter.) (https://www.manualslib.com/manual/246619/Panasonic-Viera-Tc-P50c2.html?page=50#manual ) See some converters here: https://www.amazon.com/DA-Converters/b?ie=UTF8&node=3224445011
I hope this helps.