The sound of waves is caused by interference in the ignition system, problems with the audio settings, or faults in the audio equipment itself.

1. The sound of waves is caused by interference in the ignition system. This sound is produced when the ignition system creates electromagnetic interference with audio equipment. You can try installing a filter on the audio device's power cord or plugging the audio device into a different outlet to reduce electromagnetic interference.

2. Other speakers do not sound because of audio setting issues. You can check the audio settings of BYD G6 to ensure that all speakers are turned on and set to the correct output mode.

3. If the sound persists after turning off the engine, then the audio equipment itself is faulty. You can try using other audio devices to troubleshoot this issue.