I assume this is just when using the soundbar, and not if you use the TVs built-in speakers?
Digital processing can introduce a delay. The TV itself may do this. Connecting the soundbar directly to the cable TV box (if possible) can minimize this - but you may lose some volume/power control unless the soundbar has its own remote control. Also, if you use other things (game console, etc.) connected through the TV, you would need some other way to connect the soundbar to these different audio sources.
If you connect any part of the audio path wirelessly, especially something digital like Bluetooth, this can introduce a delay. Newer technologies like APTX low-latency can help, if available. Analog wireless (RF) has less delay. The least delay would likely be an analog wired connection.
Some TVs have a "game mode" to reduce lag when playing games on a console game. This bypasses some processing to minimize the delay in the video and audio. It may help to turn this on (you lose some other effects that may also change picture quality or available features). Alternately, if you have game mode turned on, it may help to turn it off, which might introduce more delay in the video, making it and the audio match up better.
