Streaming TV audio to Android device through Wi-Fi
It's just not possible.
Setting your smartphone aside for a moment, even assuming you have an extraordinarily stable WiFi signal, transmitting audio over WiFi would introduce enough latency to ruin your day.
In some cases, the transmitter side could introduce a delay on the video stream to compensate for the audio delay (forget about gaming or anything like that in this case). Your LG TV might have a &quot;AV Sync Adjustment&quot; option, but even then, this might not be enough.
In any case, your TV will not cast audio by itself and here we come to your &quot;second phone&quot; as transmitter. The phone is unlikely to have a line-level audio input, and even if you still have a phone with a 3.5mm audio jack, that is a microphone input. To make things worse, this input is MONO.
Your

I don't want to buy more devices

rules out any other type of audio input adapter for your transmitter phone. I'd say this is a dead end.