IF your TV has analog audio out through RCA jacks or a 3.5mm stereo phone jack (somewhat rarer on today's TVs), those are generally variable outputs controlled by the TV's volume control.
If your only audio output from the TV is digital (Toslink Optical or Coaxial), those are most often fixed-level and the volume will need to be controlled either by the soundbar's remote control or a universal/programmable remote to control both.
Steve