No. This cannot be done.
Your MacBook doesn't have HDMI ports; they are Thunderbolt 3 ports meaning the display signal you're getting from it is DisplayPort. So, they will not support ARC* (HDMI technology) as mentioned in the comments. Secondly, the DisplayPort signal is one way; it's video out.
The Mac isn't designed to be an monitor with audio/video capabilities; it's not an HDMI endpoint device, it's the source. This would be like asking your TV to send audio or video to your XBox or DVD player.
Now, this doesn't mean you can't input sound into your Mac; obviously it can because it has a line input port. What you need to do is extract the audio channel from the HDMI with an HDMI Audio De-Embedder/Extractor then take the audio via a 3.5mm cable and input it into the Mac. You can then use a product like Audio Hijack or Soundflower to route the audio from the Line-In to the speakers.
While this is obviously possible, sending this into a computer will inherently introduce a audio delay as it has to be processed by the Mac. Remember, your Mac isn't an amplifier where the audio is simply boosted, it's being processed. Even a 1/4 second delay will be very noticeable and quite distracting. Since you can't introduce a video delay on your LG, there will be no way to overcome this.
Buy a cheap set of speakers or a soundbar for the TV. A lot of the Bluetooth speakers have a AUX input and the LG TVs have a 3.5mm output. If not, you can always get the HDMI audio extractor.
* Audio Return Channel. Wikipedia: HDMI Ethernet and Audio Return Channel.